diff --git a/src/cmd/asm/internal/asm/asm.go b/src/cmd/asm/internal/asm/asm.go
index e098961..4c63b51 100644
--- a/src/cmd/asm/internal/asm/asm.go
+++ b/src/cmd/asm/internal/asm/asm.go
@@ -27,15 +27,18 @@
 		case '5':
 			if !arch.ARMConditionCodes(prog, cond) {
 				p.errorf("unrecognized condition code .%q", cond)
+				return
 			}
 
 		case '7':
 			if !arch.ARM64Suffix(prog, cond) {
 				p.errorf("unrecognized suffix .%q", cond)
+				return
 			}
 
 		default:
 			p.errorf("unrecognized suffix .%q", cond)
+			return
 		}
 	}
 	if p.firstProg == nil {
@@ -49,6 +52,7 @@
 		for _, label := range p.pendingLabels {
 			if p.labels[label] != nil {
 				p.errorf("label %q multiply defined", label)
+				return
 			}
 			p.labels[label] = prog
 		}
@@ -67,8 +71,7 @@
 func (p *Parser) validateSymbol(pseudo string, addr *obj.Addr, offsetOk bool) {
 	if addr.Name != obj.NAME_EXTERN && addr.Name != obj.NAME_STATIC || addr.Scale != 0 || addr.Reg != 0 {
 		p.errorf("%s symbol %q must be a symbol(SB)", pseudo, symbolName(addr))
-	}
-	if !offsetOk && addr.Offset != 0 {
+	} else if !offsetOk && addr.Offset != 0 {
 		p.errorf("%s symbol %q must not be offset from SB", pseudo, symbolName(addr))
 	}
 }
@@ -144,6 +147,7 @@
 		// There is an argument size. It must be a minus sign followed by a non-negative integer literal.
 		if len(op) != 2 || op[0].ScanToken != '-' || op[1].ScanToken != scanner.Int {
 			p.errorf("TEXT %s: argument size must be of form -integer", name)
+			return
 		}
 		argSize = p.positiveAtoi(op[1].String())
 	}
@@ -195,11 +199,13 @@
 		// OK
 	default:
 		p.errorf("DATA value must be an immediate constant or address")
+		return
 	}
 
 	// The addresses must not overlap. Easiest test: require monotonicity.
 	if lastAddr, ok := p.dataAddr[name]; ok && nameAddr.Offset < lastAddr {
 		p.errorf("overlapping DATA entry for %s", name)
+		return
 	}
 	p.dataAddr[name] = nameAddr.Offset + int64(scale)
 
@@ -340,6 +346,7 @@
 			reg, ok := p.arch.RegisterNumber("R", int16(reg))
 			if !ok {
 				p.errorf("bad register number %d", reg)
+				return
 			}
 			prog.Reg = reg
 			break
@@ -390,6 +397,7 @@
 		prog.To = a[0]
 	default:
 		p.errorf("cannot assemble jump %+v", target)
+		return
 	}
 
 	p.append(prog, cond, true)
@@ -400,9 +408,9 @@
 		targetProg := p.labels[patch.label]
 		if targetProg == nil {
 			p.errorf("undefined label %s", patch.label)
-		} else {
-			p.branch(patch.prog, targetProg)
+			return
 		}
+		p.branch(patch.prog, targetProg)
 	}
 	p.toPatch = p.toPatch[:0]
 }
@@ -468,6 +476,7 @@
 					break
 				}
 				p.errorf("unrecognized addressing for %s", obj.Aconv(op))
+				return
 			}
 			if arch.IsARMFloatCmp(op) {
 				prog.From = a[0]
@@ -506,6 +515,7 @@
 				prog.To = a[1]
 				if a[2].Type != obj.TYPE_REG {
 					p.errorf("invalid addressing modes for third operand to %s instruction, must be register", obj.Aconv(op))
+					return
 				}
 				prog.RegTo2 = a[2].Reg
 				break
@@ -541,9 +551,11 @@
 				prog.To = a[2]
 			default:
 				p.errorf("invalid addressing modes for %s instruction", obj.Aconv(op))
+				return
 			}
 		default:
 			p.errorf("TODO: implement three-operand instructions for this architecture")
+			return
 		}
 	case 4:
 		if p.arch.Thechar == '5' && arch.IsARMMULA(op) {
@@ -577,6 +589,7 @@
 			break
 		}
 		p.errorf("can't handle %s instruction with 4 operands", obj.Aconv(op))
+		return
 	case 5:
 		if p.arch.Thechar == '9' && arch.IsPPC64RLD(op) {
 			// Always reg, reg, con, con, reg.  (con, con is a 'mask').
@@ -598,6 +611,7 @@
 			break
 		}
 		p.errorf("can't handle %s instruction with 5 operands", obj.Aconv(op))
+		return
 	case 6:
 		if p.arch.Thechar == '5' && arch.IsARMMRC(op) {
 			// Strange special case: MCR, MRC.
@@ -621,6 +635,7 @@
 		fallthrough
 	default:
 		p.errorf("can't handle %s instruction with %d operands", obj.Aconv(op), len(a))
+		return
 	}
 
 	p.append(prog, cond, true)
diff --git a/src/cmd/asm/internal/asm/operand_test.go b/src/cmd/asm/internal/asm/operand_test.go
index d196574..0f8271b5 100644
--- a/src/cmd/asm/internal/asm/operand_test.go
+++ b/src/cmd/asm/internal/asm/operand_test.go
@@ -293,6 +293,7 @@
 	{"[R0,R1,g,R15", ""}, // Issue 11764 - asm hung parsing ']' missing register lists.
 	{"[):[o-FP", ""},     // Issue 12469 - there was no infinite loop for ARM; these are just sanity checks.
 	{"[):[R0-FP", ""},
+	{"(", ""}, // Issue 12466 - backed up before beginning of line.
 }
 
 var ppc64OperandTests = []operandTest{
diff --git a/src/cmd/asm/internal/asm/parse.go b/src/cmd/asm/internal/asm/parse.go
index 7d81d4b..b38b53d 100644
--- a/src/cmd/asm/internal/asm/parse.go
+++ b/src/cmd/asm/internal/asm/parse.go
@@ -155,6 +155,7 @@
 					// Remember this location so we can swap the operands below.
 					if colon >= 0 {
 						p.errorf("invalid ':' in operand")
+						return true
 					}
 					colon = len(operands)
 				}
@@ -338,8 +339,13 @@
 	case scanner.Int, scanner.Float, scanner.String, scanner.Char, '+', '-', '~':
 		haveConstant = true
 	case '(':
-		// Could be parenthesized expression or (R).
-		rname := p.next().String()
+		// Could be parenthesized expression or (R). Must be something, though.
+		tok := p.next()
+		if tok.ScanToken == scanner.EOF {
+			p.errorf("missing right parenthesis")
+			return false
+		}
+		rname := tok.String()
 		p.back()
 		haveConstant = !p.atStartOfRegister(rname)
 		if !haveConstant {
@@ -361,6 +367,7 @@
 		if p.have(scanner.String) {
 			if prefix != '$' {
 				p.errorf("string constant must be an immediate")
+				return false
 			}
 			str, err := strconv.Unquote(p.get(scanner.String).String())
 			if err != nil {
@@ -952,7 +959,11 @@
 }
 
 func (p *Parser) back() {
-	p.inputPos--
+	if p.inputPos == 0 {
+		p.errorf("internal error: backing up before BOL")
+	} else {
+		p.inputPos--
+	}
 }
 
 func (p *Parser) peek() lex.ScanToken {
diff --git a/src/cmd/asm/internal/asm/pseudo_test.go b/src/cmd/asm/internal/asm/pseudo_test.go
index df1adc5..ee13b72 100644
--- a/src/cmd/asm/internal/asm/pseudo_test.go
+++ b/src/cmd/asm/internal/asm/pseudo_test.go
@@ -35,6 +35,8 @@
 		{"TEXT", "%", "expect two or three operands for TEXT"},
 		{"TEXT", "1, 1", "TEXT symbol \"<erroneous symbol>\" must be a symbol(SB)"},
 		{"TEXT", "$\"foo\", 0, $1", "TEXT symbol \"<erroneous symbol>\" must be a symbol(SB)"},
+		{"TEXT", "$0É:0, 0, $1", "expected EOF, found É"},   // Issue #12467.
+		{"TEXT", "$:0:(SB, 0, $1", "expected '(', found 0"}, // Issue 12468.
 		{"FUNCDATA", "", "expect two operands for FUNCDATA"},
 		{"FUNCDATA", "(SB ", "expect two operands for FUNCDATA"},
 		{"DATA", "", "expect two operands for DATA"},