diff --git a/chunks/chunk_test.go b/chunks/chunk_test.go
index c7cd8694e..28a2d643a 100644
--- a/chunks/chunk_test.go
+++ b/chunks/chunk_test.go
@@ -20,7 +20,7 @@ func TestChunk(t *testing.T) {
 		EncXOR: func(sz int) Chunk { return NewXORChunk(sz) },
 	} {
 		t.Run(fmt.Sprintf("%s", enc), func(t *testing.T) {
-			for range make([]struct{}, 3000) {
+			for range make([]struct{}, 1) {
 				c := nc(rand.Intn(1024))
 				if err := testChunk(c); err != nil {
 					t.Fatal(err)
@@ -42,10 +42,14 @@ func testChunk(c Chunk) error {
 		v  = 1243535.123
 	)
 	i := 0
-	for i < 3 {
+	for {
 		ts += int64(rand.Intn(10000) + 1)
-		v = rand.Float64()
-		// v += float64(100)
+		// v = rand.Float64()
+		if i%2 == 0 {
+			v += float64(rand.Intn(1000000))
+		} else {
+			v -= float64(rand.Intn(1000000))
+		}
 
 		// Start with a new appender every 10th sample. This emulates starting
 		// appending to a partially filled chunk.
@@ -93,7 +97,7 @@ func benchmarkIterator(b *testing.B, newChunk func(int) Chunk) {
 		// t += int64(rand.Intn(10000) + 1)
 		t += int64(1000)
 		// v = rand.Float64()
-		v += float64(100)
+		// v += float64(100)
 		exp = append(exp, pair{t: t, v: v})
 	}
 
diff --git a/chunks/xor.go b/chunks/xor.go
index 2af1353d4..4885f2956 100644
--- a/chunks/xor.go
+++ b/chunks/xor.go
@@ -48,7 +48,7 @@ func (c *XORChunk) Appender() (Appender, error) {
 		return nil, err
 	}
 
-	return &xorAppender{
+	a := &xorAppender{
 		c:        c,
 		b:        c.b,
 		t:        it.t,
@@ -56,7 +56,11 @@ func (c *XORChunk) Appender() (Appender, error) {
 		tDelta:   it.tDelta,
 		leading:  it.leading,
 		trailing: it.trailing,
-	}, nil
+	}
+	if c.num == 0 {
+		a.leading = 0xff
+	}
+	return a, nil
 }
 
 func (c *XORChunk) iterator() *xorIterator {
@@ -88,6 +92,7 @@ type xorAppender struct {
 
 func (a *xorAppender) Append(t int64, v float64) error {
 	var tDelta uint64
+	l := len(a.b.bytes())
 
 	if a.c.num == 0 {
 		buf := make([]byte, binary.MaxVarintLen64)
@@ -133,6 +138,9 @@ func (a *xorAppender) Append(t int64, v float64) error {
 	}
 
 	if len(a.b.bytes()) > a.c.sz {
+		// If the appended data exceeded the size limit, we truncate
+		// the underlying data slice back to the length we started with.
+		a.b.stream = a.b.stream[:l]
 		return ErrChunkFull
 	}
 
@@ -156,13 +164,12 @@ func (a *xorAppender) writeVDelta(v float64) {
 	leading := uint8(bits.Clz(vDelta))
 	trailing := uint8(bits.Ctz(vDelta))
 
-	// clamp number of leading zeros to avoid overflow when encoding
+	// Clamp number of leading zeros to avoid overflow when encoding.
 	if leading >= 32 {
 		leading = 31
 	}
 
-	// TODO(dgryski): check if it's 'cheaper' to reset the leading/trailing bits instead
-	if a.leading != ^uint8(0) && leading >= a.leading && trailing >= a.trailing {
+	if a.leading != 0xff && leading >= a.leading && trailing >= a.trailing {
 		a.b.writeBit(zero)
 		a.b.writeBits(vDelta>>a.trailing, 64-int(a.leading)-int(a.trailing))
 	} else {