diff --git a/git-codereview/branch.go b/git-codereview/branch.go
index 9c9d8e7..40288c7 100644
--- a/git-codereview/branch.go
+++ b/git-codereview/branch.go
@@ -7,7 +7,6 @@
 import (
 	"bytes"
 	"fmt"
-	"os"
 	"os/exec"
 	"regexp"
 	"strings"
@@ -34,10 +33,24 @@
 	return &Branch{Name: name}
 }
 
+// DetachedHead reports whether branch b corresponds to a detached HEAD
+// (does not have a real branch name).
+func (b *Branch) DetachedHead() bool {
+	return b.Name == "HEAD"
+}
+
 // OriginBranch returns the name of the origin branch that branch b tracks.
 // The returned name is like "origin/master" or "origin/dev.garbage" or
 // "origin/release-branch.go1.4".
 func (b *Branch) OriginBranch() string {
+	if b.DetachedHead() {
+		// Detached head mode.
+		// "origin/HEAD" is clearly false, but it should be easy to find when it
+		// appears in other commands. Really any caller of OriginBranch
+		// should check for detached head mode.
+		return "origin/HEAD"
+	}
+
 	if b.originBranch != "" {
 		return b.originBranch
 	}
@@ -52,7 +65,7 @@
 		b.originBranch = "origin/master"
 		return b.originBranch
 	}
-	fmt.Fprintf(os.Stderr, "%v\n%s\n", commandString(argv[0], argv[1:]), out)
+	fmt.Fprintf(stderr(), "%v\n%s\n", commandString(argv[0], argv[1:]), out)
 	dief("%v", err)
 	panic("not reached")
 }
@@ -99,6 +112,10 @@
 	}
 	b.loadedPending = true
 
+	if b.DetachedHead() {
+		return
+	}
+
 	const numField = 5
 	all := getOutput("git", "log", "--format=format:%H%x00%h%x00%P%x00%s%x00%B%x00", b.OriginBranch()+".."+b.Name)
 	fields := strings.Split(all, "\x00")
@@ -192,8 +209,20 @@
 
 func LocalBranches() []*Branch {
 	var branches []*Branch
+	current := CurrentBranch()
 	for _, s := range getLines("git", "branch", "-q") {
-		s = strings.TrimPrefix(strings.TrimSpace(s), "* ")
+		s = strings.TrimSpace(s)
+		if strings.HasPrefix(s, "* ") {
+			// * marks current branch in output.
+			// Normally the current branch has a name like any other,
+			// but in detached HEAD mode the branch listing shows
+			// a localized (translated) textual description instead of
+			// a branch name. Avoid language-specific differences
+			// by using CurrentBranch().Name for the current branch.
+			// It detects detached HEAD mode in a more portable way.
+			// (git rev-parse --abbrev-ref HEAD returns 'HEAD').
+			s = current.Name
+		}
 		branches = append(branches, &Branch{Name: s})
 	}
 	return branches
diff --git a/git-codereview/branch_test.go b/git-codereview/branch_test.go
index 10949fe..1c18cfa 100644
--- a/git-codereview/branch_test.go
+++ b/git-codereview/branch_test.go
@@ -4,7 +4,10 @@
 
 package main
 
-import "testing"
+import (
+	"reflect"
+	"testing"
+)
 
 func TestCurrentBranch(t *testing.T) {
 	gt := newGitTest(t)
@@ -34,6 +37,10 @@
 	write(t, gt.client+"/file", "i made another change")
 	trun(t, gt.client, "git", "commit", "-a", "-m", "My other change line.\n\nChange-Id: I1123456789abcdef0123456789abcdef\n")
 	checkCurrentBranch(t, "newdev", "origin/dev.branch", true, true, "I1123456789abcdef0123456789abcdef", "My other change line.")
+
+	t.Logf("detached head mode")
+	trun(t, gt.client, "git", "checkout", "HEAD^0")
+	checkCurrentBranch(t, "HEAD", "origin/HEAD", false, false, "", "")
 }
 
 func checkCurrentBranch(t *testing.T, name, origin string, isLocal, hasPending bool, changeID, subject string) {
@@ -57,3 +64,30 @@
 		t.Errorf("b.Subject() = %q, want %q", x, subject)
 	}
 }
+
+func TestLocalBranches(t *testing.T) {
+	gt := newGitTest(t)
+	defer gt.done()
+
+	t.Logf("on master")
+	checkLocalBranches(t, "master")
+
+	t.Logf("on dev branch")
+	trun(t, gt.client, "git", "checkout", "-b", "newbranch")
+	checkLocalBranches(t, "master", "newbranch")
+
+	t.Logf("detached head mode")
+	trun(t, gt.client, "git", "checkout", "HEAD^0")
+	checkLocalBranches(t, "HEAD", "master", "newbranch")
+}
+
+func checkLocalBranches(t *testing.T, want ...string) {
+	var names []string
+	branches := LocalBranches()
+	for _, b := range branches {
+		names = append(names, b.Name)
+	}
+	if !reflect.DeepEqual(names, want) {
+		t.Errorf("LocalBranches() = %v, want %v", names, want)
+	}
+}
diff --git a/git-codereview/change.go b/git-codereview/change.go
index f7b8ec1..4281b6a 100644
--- a/git-codereview/change.go
+++ b/git-codereview/change.go
@@ -19,7 +19,7 @@
 	flags.BoolVar(&changeQuick, "q", false, "do not edit pending commit msg")
 	flags.Parse(args)
 	if len(flags.Args()) > 1 {
-		fmt.Fprintf(os.Stderr, "Usage: %s change %s [branch]\n", os.Args[0], globalFlags)
+		fmt.Fprintf(stderr(), "Usage: %s change %s [branch]\n", os.Args[0], globalFlags)
 		os.Exit(2)
 
 	}
@@ -71,6 +71,10 @@
 var testCommitMsg string
 
 func commitChanges(amend bool) {
+	// git commit will run the gofmt hook.
+	// Run it now to give a better error (won't show a git commit command failing).
+	hookGofmt()
+
 	if HasUnstagedChanges() && !HasStagedChanges() && !changeAuto {
 		printf("warning: unstaged changes and no staged changes; use 'git add' or 'git change -a'")
 	}
@@ -102,6 +106,13 @@
 }
 
 func checkoutOrCreate(target string) {
+	if strings.ToUpper(target) == "HEAD" {
+		// Git gets very upset and confused if you 'git change head'
+		// on systems with case-insensitive file names: the branch
+		// head conflicts with the usual HEAD.
+		dief("invalid branch name %q: ref name HEAD is reserved for git.", target)
+	}
+
 	// If local branch exists, check it out.
 	for _, b := range LocalBranches() {
 		if b.Name == target {
diff --git a/git-codereview/change_test.go b/git-codereview/change_test.go
index 4a24c6a..daf06e3 100644
--- a/git-codereview/change_test.go
+++ b/git-codereview/change_test.go
@@ -36,6 +36,14 @@
 	testRan(t, "git checkout -q -t -b dev.branch origin/dev.branch")
 }
 
+func TestChangeHEAD(t *testing.T) {
+	gt := newGitTest(t)
+	defer gt.done()
+
+	testMainDied(t, "change", "HeAd")
+	testPrintedStderr(t, "invalid branch name \"HeAd\": ref name HEAD is reserved for git")
+}
+
 func TestMessageRE(t *testing.T) {
 	for _, c := range []struct {
 		in   string
diff --git a/git-codereview/gofmt.go b/git-codereview/gofmt.go
index 4cde005..648b795 100644
--- a/git-codereview/gofmt.go
+++ b/git-codereview/gofmt.go
@@ -8,7 +8,6 @@
 	"bytes"
 	"flag"
 	"fmt"
-	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -22,7 +21,7 @@
 	flags.BoolVar(&gofmtList, "l", false, "list files that need to be formatted")
 	flags.Parse(args)
 	if len(flag.Args()) > 0 {
-		fmt.Fprintf(os.Stderr, "Usage: %s gofmt %s [-l]\n", globalFlags, os.Args[0])
+		fmt.Fprintf(stderr(), "Usage: %s gofmt %s [-l]\n", globalFlags, os.Args[0])
 		os.Exit(2)
 	}
 
@@ -33,12 +32,9 @@
 
 	files, stderr := runGofmt(f)
 	if gofmtList {
-		var out io.Writer = os.Stdout
-		if stdoutTrap != nil {
-			out = stdoutTrap
-		}
+		w := stdout()
 		for _, file := range files {
-			fmt.Fprintf(out, "%s\n", file)
+			fmt.Fprintf(w, "%s\n", file)
 		}
 	}
 	if stderr != "" {
diff --git a/git-codereview/gofmt_test.go b/git-codereview/gofmt_test.go
index a4b884c..8cd3d81 100644
--- a/git-codereview/gofmt_test.go
+++ b/git-codereview/gofmt_test.go
@@ -46,10 +46,10 @@
 	testPrintedStdout(t, "bad.go\n", "!good.go", "!test/bad", "test/bench/bad.go")
 
 	testMain(t, "gofmt")
-	testPrintedStdout(t, "!.go") // no output
+	testNoStdout(t)
 
 	testMain(t, "gofmt", "-l")
-	testPrintedStdout(t, "!.go") // no output
+	testNoStdout(t)
 
 	write(t, gt.client+"/bad.go", badGo)
 	write(t, gt.client+"/broken.go", brokenGo)
@@ -146,7 +146,7 @@
 
 	// Reformat in place.
 	testMainDied(t, "gofmt")
-	testPrintedStdout(t, "!.go")
+	testNoStdout(t)
 	testPrintedStderr(t, wantErr...)
 
 	// Read files to make sure unstaged did not bleed into staged.
@@ -166,6 +166,6 @@
 
 	// Check that gofmt -l still shows the errors.
 	testMainDied(t, "gofmt", "-l")
-	testPrintedStdout(t, "!.go")
+	testNoStdout(t)
 	testPrintedStderr(t, wantErr...)
 }
diff --git a/git-codereview/hook.go b/git-codereview/hook.go
index 165e6e8..921fd91 100644
--- a/git-codereview/hook.go
+++ b/git-codereview/hook.go
@@ -156,6 +156,11 @@
 func hookPreCommit(args []string) {
 	// Prevent commits to master branches.
 	b := CurrentBranch()
+	if b.DetachedHead() {
+		// This is an internal commit such as during git rebase.
+		// Don't die, and don't force gofmt.
+		return
+	}
 	if !b.IsLocalOnly() {
 		dief("cannot commit on %s branch", b.Name)
 	}
diff --git a/git-codereview/hook_test.go b/git-codereview/hook_test.go
index 6cd822f..15b04a0 100644
--- a/git-codereview/hook_test.go
+++ b/git-codereview/hook_test.go
@@ -43,7 +43,7 @@
 	write(t, gt.client+"/empty.txt", "\n\n# just a file with\n# comments\n")
 	testMainDied(t, "hook-invoke", "commit-msg", gt.client+"/empty.txt")
 	const want = "git-codereview: empty commit message\n"
-	if got := stderr.String(); got != want {
+	if got := testStderr.String(); got != want {
 		t.Fatalf("unexpected output:\ngot: %q\nwant: %q", got, want)
 	}
 }
@@ -81,6 +81,55 @@
 		"gofmt reported errors:", "broken.go")
 }
 
+func TestHookChangeGofmt(t *testing.T) {
+	// During git change, we run the gofmt check before invoking commit,
+	// so we should not see the line about 'git commit' failing.
+	// That is, the failure should come from git change, not from
+	// the commit hook.
+	gt := newGitTest(t)
+	defer gt.done()
+	gt.work(t)
+
+	// Write out a non-Go file.
+	write(t, gt.client+"/bad.go", badGo)
+	trun(t, gt.client, "git", "add", ".")
+
+	t.Logf("invoking commit hook explicitly")
+	testMainDied(t, "hook-invoke", "pre-commit")
+	testPrintedStderr(t, "gofmt needs to format these files (run 'git gofmt'):", "bad.go")
+
+	t.Logf("change without hook installed")
+	testCommitMsg = "foo: msg"
+	testMainDied(t, "change")
+	testPrintedStderr(t, "gofmt needs to format these files (run 'git gofmt'):", "bad.go", "!running: git")
+
+	t.Logf("change with hook installed")
+	restore := testInstallHook(t, gt)
+	defer restore()
+	testCommitMsg = "foo: msg"
+	testMainDied(t, "change")
+	testPrintedStderr(t, "gofmt needs to format these files (run 'git gofmt'):", "bad.go", "!running: git")
+}
+
+func TestHookPreCommitDetachedHead(t *testing.T) {
+	// If we're in detached head mode, something special is going on,
+	// like git rebase. We disable the gofmt-checking precommit hook,
+	// since we expect it would just get in the way at that point.
+	// (It also used to crash.)
+
+	gt := newGitTest(t)
+	defer gt.done()
+	gt.work(t)
+
+	write(t, gt.client+"/bad.go", badGo)
+	trun(t, gt.client, "git", "add", ".")
+	trun(t, gt.client, "git", "checkout", "HEAD^0")
+
+	testMain(t, "hook-invoke", "pre-commit")
+	testNoStdout(t)
+	testNoStderr(t)
+}
+
 func TestHookPreCommitUnstaged(t *testing.T) {
 	gt := newGitTest(t)
 	defer gt.done()
@@ -181,17 +230,25 @@
 	}
 }
 
+func testInstallHook(t *testing.T, gt *gitTest) (restore func()) {
+	trun(t, gt.pwd, "go", "build", "-o", gt.client+"/git-codereview")
+	path := os.Getenv("PATH")
+	os.Setenv("PATH", gt.client+string(filepath.ListSeparator)+path)
+	gt.removeStubHooks()
+	testMain(t, "hooks") // install hooks
+
+	return func() {
+		os.Setenv("PATH", path)
+	}
+}
+
 func TestHookCommitMsgFromGit(t *testing.T) {
 	gt := newGitTest(t)
 	defer gt.done()
 
-	trun(t, gt.pwd, "go", "build", "-o", gt.client+"/git-codereview")
-	path := os.Getenv("PATH")
-	defer os.Setenv("PATH", path)
-	os.Setenv("PATH", gt.client+string(filepath.ListSeparator)+path)
+	restore := testInstallHook(t, gt)
+	defer restore()
 
-	gt.removeStubHooks()
-	testMain(t, "hooks") // install hooks
 	testMain(t, "change", "mybranch")
 	write(t, gt.client+"/file", "more data")
 	trun(t, gt.client, "git", "add", "file")
diff --git a/git-codereview/mail.go b/git-codereview/mail.go
index 01dd4b8..c06a784 100644
--- a/git-codereview/mail.go
+++ b/git-codereview/mail.go
@@ -22,7 +22,7 @@
 	flags.Var(ccList, "cc", "comma-separated list of people to CC:")
 
 	flags.Usage = func() {
-		fmt.Fprintf(os.Stderr, "Usage: %s mail %s [-r reviewer,...] [-cc mail,...]\n", os.Args[0], globalFlags)
+		fmt.Fprintf(stderr(), "Usage: %s mail %s [-r reviewer,...] [-cc mail,...]\n", os.Args[0], globalFlags)
 	}
 	flags.Parse(args)
 	if len(flags.Args()) != 0 {
diff --git a/git-codereview/pending.go b/git-codereview/pending.go
index e4e2a96..a90f6ce 100644
--- a/git-codereview/pending.go
+++ b/git-codereview/pending.go
@@ -52,7 +52,7 @@
 	flags.BoolVar(&pendingLocal, "l", false, "use only local information - no network operations")
 	flags.Parse(args)
 	if len(flags.Args()) > 0 {
-		fmt.Fprintf(os.Stderr, "Usage: %s pending %s [-l]\n", os.Args[0], globalFlags)
+		fmt.Fprintf(stderr(), "Usage: %s pending %s [-l]\n", os.Args[0], globalFlags)
 		os.Exit(2)
 	}
 
@@ -219,11 +219,7 @@
 		fmt.Fprintf(&buf, "\n")
 	}
 
-	if stdoutTrap != nil {
-		stdoutTrap.Write(buf.Bytes())
-	} else {
-		os.Stdout.Write(buf.Bytes())
-	}
+	stdout().Write(buf.Bytes())
 }
 
 // errors returns any errors that should be displayed
diff --git a/git-codereview/pending_test.go b/git-codereview/pending_test.go
index b1030e2..68c0f00 100644
--- a/git-codereview/pending_test.go
+++ b/git-codereview/pending_test.go
@@ -241,7 +241,7 @@
 	want = strings.TrimPrefix(want, "\n")
 
 	testMain(t, "pending")
-	out := stdout.Bytes()
+	out := testStdout.Bytes()
 
 	out = regexp.MustCompile(`\b[0-9a-f]{7}\b`).ReplaceAllLiteral(out, []byte("REVHASH"))
 	out = regexp.MustCompile(`\b127\.0\.0\.1:\d+\b`).ReplaceAllLiteral(out, []byte("127.0.0.1:PORT"))
diff --git a/git-codereview/review.go b/git-codereview/review.go
index a2a60fb..fe8867e 100644
--- a/git-codereview/review.go
+++ b/git-codereview/review.go
@@ -29,7 +29,7 @@
 func initFlags() {
 	flags = flag.NewFlagSet("", flag.ExitOnError)
 	flags.Usage = func() {
-		fmt.Fprintf(os.Stderr, usage, os.Args[0], os.Args[0])
+		fmt.Fprintf(stderr(), usage, os.Args[0], os.Args[0])
 	}
 	flags.Var(verbose, "v", "report git commands")
 	flags.BoolVar(noRun, "n", false, "print but do not run commands")
@@ -117,7 +117,7 @@
 	command, args := os.Args[1], os.Args[2:]
 
 	if command == "help" {
-		fmt.Fprintf(os.Stdout, help, os.Args[0])
+		fmt.Fprintf(stdout(), help, os.Args[0])
 		return
 	}
 
@@ -148,7 +148,7 @@
 func expectZeroArgs(args []string, command string) {
 	flags.Parse(args)
 	if len(flags.Args()) > 0 {
-		fmt.Fprintf(os.Stderr, "Usage: %s %s %s\n", os.Args[0], command, globalFlags)
+		fmt.Fprintf(stderr(), "Usage: %s %s %s\n", os.Args[0], command, globalFlags)
 		os.Exit(2)
 	}
 }
@@ -158,7 +158,7 @@
 		if *verbose == 0 {
 			// If we're not in verbose mode, print the command
 			// before dying to give context to the failure.
-			fmt.Fprintln(os.Stderr, commandString(command, args))
+			fmt.Fprintf(stderr(), "(running: %s)\n", commandString(command, args))
 		}
 		dief("%v", err)
 	}
@@ -172,7 +172,7 @@
 
 func runDirErr(dir, command string, args ...string) error {
 	if *verbose > 0 || *noRun {
-		fmt.Fprintln(os.Stderr, commandString(command, args))
+		fmt.Fprintln(stderr(), commandString(command, args))
 	}
 	if *noRun {
 		return nil
@@ -182,14 +182,8 @@
 	}
 	cmd := exec.Command(command, args...)
 	cmd.Stdin = os.Stdin
-	cmd.Stdout = os.Stdout
-	if stdoutTrap != nil {
-		cmd.Stdout = stdoutTrap
-	}
-	cmd.Stderr = os.Stderr
-	if stderrTrap != nil {
-		cmd.Stderr = stderrTrap
-	}
+	cmd.Stdout = stdout()
+	cmd.Stderr = stderr()
 	return cmd.Run()
 }
 
@@ -201,7 +195,7 @@
 func getOutput(command string, args ...string) string {
 	s, err := getOutputErr(command, args...)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "%v\n%s\n", commandString(command, args), s)
+		fmt.Fprintf(stderr(), "%v\n%s\n", commandString(command, args), s)
 		dief("%v", err)
 	}
 	return s
@@ -215,7 +209,7 @@
 	// the git repo" commands, which is confusing if you are just trying to find
 	// out what git sync means.
 	if *verbose > 1 {
-		fmt.Fprintln(os.Stderr, commandString(command, args))
+		fmt.Fprintln(stderr(), commandString(command, args))
 	}
 	b, err := exec.Command(command, args...).CombinedOutput()
 	return string(bytes.TrimSpace(b)), err
@@ -257,12 +251,22 @@
 
 var stdoutTrap, stderrTrap *bytes.Buffer
 
-func printf(format string, args ...interface{}) {
-	w := io.Writer(os.Stderr)
-	if stderrTrap != nil {
-		w = stderrTrap
+func stdout() io.Writer {
+	if stdoutTrap != nil {
+		return stdoutTrap
 	}
-	fmt.Fprintf(w, "%s: %s\n", os.Args[0], fmt.Sprintf(format, args...))
+	return os.Stdout
+}
+
+func stderr() io.Writer {
+	if stderrTrap != nil {
+		return stderrTrap
+	}
+	return os.Stderr
+}
+
+func printf(format string, args ...interface{}) {
+	fmt.Fprintf(stderr(), "%s: %s\n", os.Args[0], fmt.Sprintf(format, args...))
 }
 
 // count is a flag.Value that is like a flag.Bool and a flag.Int.
diff --git a/git-codereview/util_test.go b/git-codereview/util_test.go
index 27ed6c9..0be544a 100644
--- a/git-codereview/util_test.go
+++ b/git-codereview/util_test.go
@@ -125,10 +125,10 @@
 }
 
 var (
-	runLog []string
-	stderr *bytes.Buffer
-	stdout *bytes.Buffer
-	died   bool
+	runLog     []string
+	testStderr *bytes.Buffer
+	testStdout *bytes.Buffer
+	died       bool
 )
 
 var mainCanDie bool
@@ -137,7 +137,7 @@
 	mainCanDie = true
 	testMain(t, args...)
 	if !died {
-		t.Fatalf("expected to die, did not\nstdout:\n%sstderr:\n%s", stdout, stderr)
+		t.Fatalf("expected to die, did not\nstdout:\n%sstderr:\n%s", testStdout, testStderr)
 	}
 }
 
@@ -157,8 +157,8 @@
 
 	defer func() {
 		runLog = runLogTrap
-		stdout = stdoutTrap
-		stderr = stderrTrap
+		testStdout = stdoutTrap
+		testStderr = stderrTrap
 
 		dieTrap = nil
 		runLogTrap = nil
@@ -174,7 +174,7 @@
 			} else {
 				msg = fmt.Sprintf("panic: %v", err)
 			}
-			t.Fatalf("%s\nstdout:\n%sstderr:\n%s", msg, stdout, stderr)
+			t.Fatalf("%s\nstdout:\n%sstderr:\n%s", msg, testStdout, testStderr)
 		}
 	}()
 
@@ -221,22 +221,22 @@
 }
 
 func testPrintedStdout(t *testing.T, messages ...string) {
-	testPrinted(t, stdout, "stdout", messages...)
+	testPrinted(t, testStdout, "stdout", messages...)
 }
 
 func testPrintedStderr(t *testing.T, messages ...string) {
-	testPrinted(t, stderr, "stderr", messages...)
+	testPrinted(t, testStderr, "stderr", messages...)
 }
 
 func testNoStdout(t *testing.T) {
-	if stdout.Len() != 0 {
-		t.Fatalf("unexpected stdout:\n%s", stdout)
+	if testStdout.Len() != 0 {
+		t.Fatalf("unexpected stdout:\n%s", testStdout)
 	}
 }
 
 func testNoStderr(t *testing.T) {
-	if stderr.Len() != 0 {
-		t.Fatalf("unexpected stderr:\n%s", stderr)
+	if testStderr.Len() != 0 {
+		t.Fatalf("unexpected stderr:\n%s", testStderr)
 	}
 }