diff --git a/s2/encode_test.go b/s2/encode_test.go
index c0b05057eb..29acebce70 100644
--- a/s2/encode_test.go
+++ b/s2/encode_test.go
@@ -87,39 +87,58 @@ func TestEncoderRegression(t *testing.T) {
 		if testing.Short() && len(data) > 10000 {
 			t.SkipNow()
 		}
+		var blocksTested bool
 		for name, opts := range testOptions(t) {
 			t.Run(name, func(t *testing.T) {
 				var buf bytes.Buffer
 				dec := NewReader(nil)
 				enc := NewWriter(&buf, opts...)
 
-				comp := Encode(make([]byte, MaxEncodedLen(len(data))), data)
-				decoded, err := Decode(nil, comp)
-				if err != nil {
-					t.Error(err)
-					return
-				}
-				if !bytes.Equal(data, decoded) {
-					t.Error("block decoder mismatch")
-					return
-				}
-				if mel := MaxEncodedLen(len(data)); len(comp) > mel {
-					t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
-					return
-				}
-				comp = EncodeBetter(make([]byte, MaxEncodedLen(len(data))), data)
-				decoded, err = Decode(nil, comp)
-				if err != nil {
-					t.Error(err)
-					return
-				}
-				if !bytes.Equal(data, decoded) {
-					t.Error("block decoder mismatch")
-					return
-				}
-				if mel := MaxEncodedLen(len(data)); len(comp) > mel {
-					t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
-					return
+				if !blocksTested {
+					comp := Encode(make([]byte, MaxEncodedLen(len(data))), data)
+					decoded, err := Decode(nil, comp)
+					if err != nil {
+						t.Error(err)
+						return
+					}
+					if !bytes.Equal(data, decoded) {
+						t.Error("block decoder mismatch")
+						return
+					}
+					if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+						t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+						return
+					}
+					comp = EncodeBetter(make([]byte, MaxEncodedLen(len(data))), data)
+					decoded, err = Decode(nil, comp)
+					if err != nil {
+						t.Error(err)
+						return
+					}
+					if !bytes.Equal(data, decoded) {
+						t.Error("block decoder mismatch")
+						return
+					}
+					if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+						t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+						return
+					}
+
+					comp = EncodeBest(make([]byte, MaxEncodedLen(len(data))), data)
+					decoded, err = Decode(nil, comp)
+					if err != nil {
+						t.Error(err)
+						return
+					}
+					if !bytes.Equal(data, decoded) {
+						t.Error("block decoder mismatch")
+						return
+					}
+					if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+						t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+						return
+					}
+					blocksTested = true
 				}
 
 				// Test writer.
@@ -143,7 +162,7 @@ func TestEncoderRegression(t *testing.T) {
 					t.Error(err)
 					return
 				}
-				comp = buf.Bytes()
+				comp := buf.Bytes()
 				if enc.pad > 0 && len(comp)%enc.pad != 0 {
 					t.Error(fmt.Errorf("wanted size to be mutiple of %d, got size %d with remainder %d", enc.pad, len(comp), len(comp)%enc.pad))
 					return
diff --git a/s2/fuzz_test.go b/s2/fuzz_test.go
new file mode 100644
index 0000000000..830d3656d6
--- /dev/null
+++ b/s2/fuzz_test.go
@@ -0,0 +1,130 @@
+//go:build go1.18
+// +build go1.18
+
+package s2
+
+import (
+	"bytes"
+	"fmt"
+	"testing"
+
+	"github.com/klauspost/compress/internal/fuzz"
+	"github.com/klauspost/compress/internal/snapref"
+)
+
+func FuzzEncodingBlocks(f *testing.F) {
+	fuzz.AddFromZip(f, "testdata/enc_regressions.zip", true, false)
+	fuzz.AddFromZip(f, "testdata/fuzz/block-corpus-raw.zip", true, testing.Short())
+	fuzz.AddFromZip(f, "testdata/fuzz/block-corpus-enc.zip", false, testing.Short())
+
+	// Fuzzing tweaks:
+	const (
+		// Max input size:
+		maxSize = 8 << 20
+	)
+
+	f.Fuzz(func(t *testing.T, data []byte) {
+		if len(data) > maxSize {
+			return
+		}
+
+		writeDst := make([]byte, MaxEncodedLen(len(data)), MaxEncodedLen(len(data))+4)
+		writeDst = append(writeDst, 1, 2, 3, 4)
+		defer func() {
+			got := writeDst[MaxEncodedLen(len(data)):]
+			want := []byte{1, 2, 3, 4}
+			if !bytes.Equal(got, want) {
+				t.Fatalf("want %v, got %v - dest modified outside cap", want, got)
+			}
+		}()
+		compDst := writeDst[:MaxEncodedLen(len(data)):MaxEncodedLen(len(data))] // Hard cap
+		decDst := make([]byte, len(data))
+		comp := Encode(compDst, data)
+		decoded, err := Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+		comp = EncodeBetter(compDst, data)
+		decoded, err = Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+
+		comp = EncodeBest(compDst, data)
+		decoded, err = Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+
+		comp = EncodeSnappy(compDst, data)
+		decoded, err = snapref.Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+		comp = EncodeSnappyBetter(compDst, data)
+		decoded, err = snapref.Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+
+		comp = EncodeSnappyBest(compDst, data)
+		decoded, err = snapref.Decode(decDst, comp)
+		if err != nil {
+			t.Error(err)
+			return
+		}
+		if !bytes.Equal(data, decoded) {
+			t.Error("block decoder mismatch")
+			return
+		}
+		if mel := MaxEncodedLen(len(data)); len(comp) > mel {
+			t.Error(fmt.Errorf("MaxEncodedLen Exceed: input: %d, mel: %d, got %d", len(data), mel, len(comp)))
+			return
+		}
+	})
+}
diff --git a/s2/testdata/fuzz/block-corpus-enc.zip b/s2/testdata/fuzz/block-corpus-enc.zip
new file mode 100644
index 0000000000..1e44ebcefb
Binary files /dev/null and b/s2/testdata/fuzz/block-corpus-enc.zip differ
diff --git a/s2/testdata/fuzz/block-corpus-raw.zip b/s2/testdata/fuzz/block-corpus-raw.zip
new file mode 100644
index 0000000000..c18dd54e72
Binary files /dev/null and b/s2/testdata/fuzz/block-corpus-raw.zip differ