diff --git a/playground/socket/socket.go b/playground/socket/socket.go
index 87abf56371..cec842e54b 100644
--- a/playground/socket/socket.go
+++ b/playground/socket/socket.go
@@ -255,25 +255,28 @@ func limiter(in <-chan *Message, p killer) <-chan *Message {
 // buffer returns a channel that wraps the given channel. It receives messages
 // from the given channel and sends them to the returned channel.
 // Message bodies are gathered over the period msgDelay and coalesced into a
-// single Message before they are passed on.
-// When the given channel is closed, buffer flushes the remaining buffered
-// messages and closes the returned channel.
+// single Message before they are passed on. Messages of the same kind are
+// coalesced; when a message of a different kind is received, any buffered
+// messages are flushed. When the given channel is closed, buffer flushes the
+// remaining buffered messages and closes the returned channel.
 func buffer(in <-chan *Message) <-chan *Message {
 	out := make(chan *Message)
 	go func() {
 		defer close(out)
-		buf := make(map[string][]byte) // [kind]buffer
-		flush := func() {
-			for kind, b := range buf {
-				if len(b) == 0 {
-					continue
+		var (
+			t     = time.NewTimer(msgDelay)
+			tc    <-chan time.Time
+			buf   []byte
+			kind  string
+			flush = func() {
+				if len(buf) == 0 {
+					return
 				}
-				out <- &Message{Kind: kind, Body: safeString(b)}
-				buf[kind] = b[:0] // recycle buffer
+				out <- &Message{Kind: kind, Body: safeString(buf)}
+				buf = buf[:0] // recycle buffer
+				kind = ""
 			}
-		}
-		t := time.NewTimer(msgDelay)
-		var tc <-chan time.Time
+		)
 		for {
 			select {
 			case m, ok := <-in:
@@ -286,11 +289,15 @@ func buffer(in <-chan *Message) <-chan *Message {
 					out <- m
 					return
 				}
-				buf[m.Kind] = append(buf[m.Kind], m.Body...)
-				if tc == nil {
-					tc = t.C
-					t.Reset(msgDelay)
+				if kind != m.Kind {
+					flush()
+					kind = m.Kind
+					if tc == nil {
+						tc = t.C
+						t.Reset(msgDelay)
+					}
 				}
+				buf = append(buf, m.Body...)
 			case <-tc:
 				flush()
 				tc = nil
diff --git a/playground/socket/socket_test.go b/playground/socket/socket_test.go
new file mode 100644
index 0000000000..5dd2815ee4
--- /dev/null
+++ b/playground/socket/socket_test.go
@@ -0,0 +1,73 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package socket
+
+import (
+	"testing"
+	"time"
+)
+
+func TestBuffer(t *testing.T) {
+	ch := make(chan *Message)
+	go func() {
+		ch <- &Message{Kind: "err", Body: "a"}
+		ch <- &Message{Kind: "err", Body: "b"}
+		ch <- &Message{Kind: "out", Body: "1"}
+		ch <- &Message{Kind: "out", Body: "2"}
+		time.Sleep(msgDelay * 2)
+		ch <- &Message{Kind: "out", Body: "3"}
+		ch <- &Message{Kind: "out", Body: "4"}
+		close(ch)
+	}()
+
+	var ms []*Message
+	for m := range buffer(ch) {
+		ms = append(ms, m)
+	}
+	if len(ms) != 3 {
+		t.Fatalf("got %v messages, want 2", len(ms))
+	}
+	if g, w := ms[0].Body, "ab"; g != w {
+		t.Errorf("message 0 body = %q, want %q", g, w)
+	}
+	if g, w := ms[1].Body, "12"; g != w {
+		t.Errorf("message 1 body = %q, want %q", g, w)
+	}
+	if g, w := ms[2].Body, "34"; g != w {
+		t.Errorf("message 2 body = %q, want %q", g, w)
+	}
+}
+
+type killRecorder chan struct{}
+
+func (k killRecorder) Kill() { close(k) }
+
+func TestLimiter(t *testing.T) {
+	ch := make(chan *Message)
+	go func() {
+		var m Message
+		for i := 0; i < msgLimit+10; i++ {
+			ch <- &m
+		}
+		ch <- &Message{Kind: "end"}
+	}()
+
+	kr := make(killRecorder)
+	n := 0
+	for m := range limiter(ch, kr) {
+		n++
+		if n > msgLimit && m.Kind != "end" {
+			t.Errorf("received non-end message after limit")
+		}
+	}
+	if n != msgLimit+1 {
+		t.Errorf("received %v messages, want %v", n, msgLimit+1)
+	}
+	select {
+	case <-kr:
+	case <-time.After(100 * time.Millisecond):
+		t.Errorf("process wasn't killed after reaching limit")
+	}
+}