diff --git a/internal/cmd/pbdump/pbdump.go b/internal/cmd/pbdump/pbdump.go
index 71912b1..62e3a27 100644
--- a/internal/cmd/pbdump/pbdump.go
+++ b/internal/cmd/pbdump/pbdump.go
@@ -19,6 +19,7 @@
 
 	"google.golang.org/protobuf/internal/encoding/pack"
 	"google.golang.org/protobuf/internal/encoding/wire"
+	"google.golang.org/protobuf/internal/errors"
 	"google.golang.org/protobuf/proto"
 	"google.golang.org/protobuf/reflect/protodesc"
 	"google.golang.org/protobuf/reflect/protoreflect"
@@ -174,7 +175,7 @@
 	n, _ := strconv.ParseInt(s[:i], 10, 32)
 	num := wire.Number(n)
 	if num < wire.MinValidNumber || wire.MaxValidNumber < num {
-		return fmt.Errorf("invalid field: %v", prefix)
+		return errors.New("invalid field: %v", prefix)
 	}
 	s = strings.TrimPrefix(s[i:], ".")
 
@@ -184,7 +185,7 @@
 	}
 	if len(s) == 0 {
 		if fs[num].kind.IsValid() {
-			return fmt.Errorf("field %v already set as %v type", prefix, fs[num].kind)
+			return errors.New("field %v already set as %v type", prefix, fs[num].kind)
 		}
 		fs[num].kind = k
 	}
@@ -195,7 +196,7 @@
 	// Verify that only messages or groups can have sub-fields.
 	k2 := fs[num].kind
 	if k2 > 0 && k2 != protoreflect.MessageKind && k2 != protoreflect.GroupKind && len(fs[num].sub) > 0 {
-		return fmt.Errorf("field %v of %v type cannot have sub-fields", prefix, k2)
+		return errors.New("field %v of %v type cannot have sub-fields", prefix, k2)
 	}
 	return nil
 }
diff --git a/internal/errors/errors.go b/internal/errors/errors.go
index e22035e..db1005d 100644
--- a/internal/errors/errors.go
+++ b/internal/errors/errors.go
@@ -7,6 +7,8 @@
 
 import (
 	"fmt"
+
+	"google.golang.org/protobuf/internal/detrand"
 )
 
 // New formats a string according to the format specifier and arguments and
@@ -22,7 +24,16 @@
 
 type prefixError struct{ s string }
 
-func (e *prefixError) Error() string { return "proto: " + e.s }
+var prefix = func() string {
+	if detrand.Bool() {
+		return "proto:  "
+	}
+	return "proto: "
+}()
+
+func (e *prefixError) Error() string {
+	return prefix + e.s
+}
 
 func InvalidUTF8(name string) error {
 	return New("field %v contains invalid UTF-8", name)
diff --git a/internal/impl/codec_tables.go b/internal/impl/codec_tables.go
index 77fc4c0..f421c78 100644
--- a/internal/impl/codec_tables.go
+++ b/internal/impl/codec_tables.go
@@ -423,7 +423,7 @@
 			}
 		}
 	}
-	panic(fmt.Errorf("invalid type: no encoder for %v %v %v/%v", fd.FullName(), fd.Cardinality(), fd.Kind(), ft))
+	panic(fmt.Sprintf("invalid type: no encoder for %v %v %v/%v", fd.FullName(), fd.Cardinality(), fd.Kind(), ft))
 }
 
 // encoderFuncsForValue returns value functions for a field, used for
@@ -547,5 +547,5 @@
 			return coderGroupValue
 		}
 	}
-	panic(fmt.Errorf("invalid field: no encoder for %v %v %v", fd.FullName(), fd.Cardinality(), fd.Kind()))
+	panic(fmt.Sprintf("invalid field: no encoder for %v %v %v", fd.FullName(), fd.Cardinality(), fd.Kind()))
 }
diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index b34e479..04e8f7c 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -145,7 +145,7 @@
 }
 func (m *extensionMap) Set(xt pref.ExtensionType, v pref.Value) {
 	if !xt.IsValidValue(v) {
-		panic(fmt.Errorf("%v: assigning invalid value", xt.TypeDescriptor().FullName()))
+		panic(fmt.Sprintf("%v: assigning invalid value", xt.TypeDescriptor().FullName()))
 	}
 	if *m == nil {
 		*m = make(map[int32]ExtensionField)
diff --git a/proto/isinit_test.go b/proto/isinit_test.go
index 72dd108..d935d3a 100644
--- a/proto/isinit_test.go
+++ b/proto/isinit_test.go
@@ -6,6 +6,7 @@
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 
 	"google.golang.org/protobuf/proto"
@@ -20,13 +21,13 @@
 	}{
 		{
 			&testpb.TestRequired{},
-			`proto: required field goproto.proto.test.TestRequired.required_field not set`,
+			`goproto.proto.test.TestRequired.required_field`,
 		},
 		{
 			&testpb.TestRequiredForeign{
 				OptionalMessage: &testpb.TestRequired{},
 			},
-			`proto: required field goproto.proto.test.TestRequired.required_field not set`,
+			`goproto.proto.test.TestRequired.required_field`,
 		},
 		{
 			&testpb.TestRequiredForeign{
@@ -35,7 +36,7 @@
 					{},
 				},
 			},
-			`proto: required field goproto.proto.test.TestRequired.required_field not set`,
+			`goproto.proto.test.TestRequired.required_field`,
 		},
 		{
 			&testpb.TestRequiredForeign{
@@ -43,7 +44,7 @@
 					1: {},
 				},
 			},
-			`proto: required field goproto.proto.test.TestRequired.required_field not set`,
+			`goproto.proto.test.TestRequired.required_field`,
 		},
 	} {
 		err := proto.IsInitialized(test.m)
@@ -51,9 +52,8 @@
 		if err != nil {
 			got = fmt.Sprintf("%q", err)
 		}
-		want := fmt.Sprintf("%q", test.want)
-		if got != want {
-			t.Errorf("IsInitialized(m):\n got: %v\nwant: %v\nMessage:\n%v", got, want, marshalText(test.m))
+		if !strings.Contains(got, test.want) {
+			t.Errorf("IsInitialized(m):\n got: %v\nwant contains: %v\nMessage:\n%v", got, test.want, marshalText(test.m))
 		}
 	}
 }
diff --git a/reflect/protodesc/file_test.go b/reflect/protodesc/file_test.go
index 5319b52..38e8b4c 100644
--- a/reflect/protodesc/file_test.go
+++ b/reflect/protodesc/file_test.go
@@ -819,7 +819,7 @@
 				]
 			}]}]
 		`),
-		wantErr: `proto: message "M.M" using proto3 semantics has conflict: "baz" with "_b_a_z_"`,
+		wantErr: `message "M.M" using proto3 semantics has conflict: "baz" with "_b_a_z_"`,
 	}, {
 		label: "proto3 message fields",
 		inDesc: mustParseFile(`