diff --git a/conn.go b/conn.go
index 0816eb1..103b074 100644
--- a/conn.go
+++ b/conn.go
@@ -123,6 +123,23 @@ type Conn struct {
 	flushing bool
 	werr     error
 
+	// wireMu serializes record emission, covering the seal and the socket write
+	// as one step.
+	//
+	// It cannot be folded into wmu. Write deliberately RELEASES wmu around each
+	// writeRecord so a slow socket does not block other writers, and guards
+	// re-entry with the flushing flag -- but that flag only excludes Write from
+	// Write. Close takes the freed wmu and reaches writeRecord through
+	// writeSized, so a close_notify could seal concurrently with a data flush.
+	//
+	// Both halves of writeRecord need the same lock, and for different reasons.
+	// The sequence number IS the AEAD nonce, so two sealers taking the same
+	// sendSeq is nonce reuse under one key -- for AES-GCM that is authentication
+	// key recovery, not a decrypt failure. And even with distinct numbers the
+	// records must reach the wire in that order, because the peer decrypts
+	// against its own monotonic counter.
+	wireMu sync.Mutex
+
 	// closeOnce guards the close_notify alert: Close may be called more than
 	// once, and a second alert would itself be the anomaly.
 	closeOnce sync.Once
@@ -217,6 +234,9 @@ func (c *Conn) Write(b []byte) (int, error) {
 }
 
 func (c *Conn) writeRecord(typ byte, payload []byte, padTo int) error {
+	c.wireMu.Lock()
+	defer c.wireMu.Unlock()
+
 	inner := make([]byte, 0, len(payload)+1)
 	inner = append(inner, payload...)
 	inner = append(inner, typ)
diff --git a/conn_race_test.go b/conn_race_test.go
new file mode 100644
index 0000000..9405c48
--- /dev/null
+++ b/conn_race_test.go
@@ -0,0 +1,210 @@
+package twiddle
+
+import (
+	"bytes"
+	"io"
+	"net"
+	"sync"
+	"testing"
+	"time"
+)
+
+// gateConn records every record handed to the socket and holds the FIRST one
+// until the test releases it. Blocking there is what forces the overlap between
+// a data flush and close_notify, rather than leaving it to chance.
+type gateConn struct {
+	net.Conn
+
+	entered chan struct{}
+	release chan struct{}
+
+	mu      sync.Mutex
+	gated   bool
+	records [][]byte
+}
+
+func newGateConn() *gateConn {
+	return &gateConn{entered: make(chan struct{}), release: make(chan struct{})}
+}
+
+func (c *gateConn) Write(b []byte) (int, error) {
+	c.mu.Lock()
+	first := !c.gated
+	c.gated = true
+	c.mu.Unlock()
+	if first {
+		close(c.entered)
+		<-c.release
+	}
+	c.mu.Lock()
+	c.records = append(c.records, append([]byte(nil), b...))
+	c.mu.Unlock()
+	return len(b), nil
+}
+
+func (c *gateConn) Close() error { return nil }
+
+func (c *gateConn) wire() []byte {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return bytes.Join(c.records, nil)
+}
+
+func (c *gateConn) count() int {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return len(c.records)
+}
+
+// replayConn serves a fixed byte stream to a peer Conn, so the recorded wire
+// can be decrypted exactly as the far end would decrypt it.
+type replayConn struct {
+	net.Conn
+	r *bytes.Reader
+}
+
+func (c *replayConn) Read(b []byte) (int, error) { return c.r.Read(b) }
+func (c *replayConn) Close() error               { return nil }
+
+func testSession(t *testing.T) *Session {
+	t.Helper()
+	s, err := DeriveSession(make([]byte, 32), make([]byte, 32), TLS_AES_128_GCM_SHA256)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return s
+}
+
+// A close_notify concurrent with a data flush must not overtake it on the wire.
+//
+// Write releases wmu around writeRecord so a slow socket cannot block other
+// writers; Close reaches writeRecord through writeSized, which takes that freed
+// wmu. Before wireMu the two ran concurrently, and because the sequence number
+// is the AEAD nonce the peer could not decrypt what came out: it counts
+// monotonically, so a record that arrives before the one it was sealed after is
+// authenticated against the wrong nonce.
+//
+// This pins the ordering half deterministically. The other half -- two sealers
+// reading the same sendSeq, which is nonce reuse under one key -- is the
+// unsynchronised increment, and is what -race reports on the same overlap.
+func TestCloseNotifyCannotOvertakeAFlush(t *testing.T) {
+	sess := testSession(t)
+	gate := newGateConn()
+	w, err := NewConn(gate, sess, true, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	payload := []byte("application data that must arrive first")
+	wrote := make(chan error, 1)
+	go func() {
+		_, err := w.Write(payload)
+		wrote <- err
+	}()
+
+	select {
+	case <-gate.entered:
+	case <-time.After(3 * time.Second):
+		t.Fatal("the data record never reached the socket")
+	}
+
+	started := make(chan struct{})
+	closed := make(chan struct{})
+	go func() {
+		close(started)
+		_ = w.Close()
+		close(closed)
+	}()
+	<-started
+
+	// The data record is still parked in the socket write, so close_notify must
+	// be waiting behind it rather than already on the wire.
+	//
+	// Proving that it did NOT happen needs a bound, and the bound has to outlast
+	// scheduling delay on a loaded machine: too short and a Close that never got
+	// to run reads as a Close that correctly waited, which passes for the wrong
+	// reason. started only proves the goroutine exists, so the loop also watches
+	// the wire itself -- without wireMu, Close's record is not blocked by the
+	// gate (only the first write is) and appears there directly, which is the
+	// symptom rather than a proxy for it.
+	deadline := time.Now().Add(2 * time.Second)
+	for time.Now().Before(deadline) {
+		if n := gate.count(); n != 0 {
+			t.Fatalf("%d records reached the wire while a flush held it", n)
+		}
+		select {
+		case <-closed:
+			t.Fatal("close_notify was emitted while a flush held the wire")
+		default:
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+
+	close(gate.release)
+	if err := <-wrote; err != nil {
+		t.Fatal(err)
+	}
+	select {
+	case <-closed:
+	case <-time.After(3 * time.Second):
+		t.Fatal("Close never returned")
+	}
+
+	// The real assertion: the peer decrypts the recorded wire with its own
+	// monotonic counter. Any reordering or nonce reuse fails to authenticate.
+	peer, err := NewConn(&replayConn{r: bytes.NewReader(gate.wire())}, sess, false, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	got := make([]byte, 0, len(payload))
+	buf := make([]byte, 512)
+	for {
+		n, err := peer.Read(buf)
+		got = append(got, buf[:n]...)
+		if err != nil {
+			if err != io.EOF {
+				t.Fatalf("peer could not decrypt the recorded wire: %v", err)
+			}
+			break
+		}
+	}
+	if !bytes.Equal(got, payload) {
+		t.Fatalf("peer decrypted %q, want %q", got, payload)
+	}
+	if n := gate.count(); n != 2 {
+		t.Fatalf("wire carries %d records, want the data record and close_notify", n)
+	}
+}
+
+// The same overlap without the gate, so -race sees the unsynchronised sendSeq
+// if wireMu is ever removed.
+func TestConcurrentWritesAndCloseAreRaceFree(t *testing.T) {
+	sess := testSession(t)
+	server, client := net.Pipe()
+	defer server.Close()
+	go func() {
+		_, _ = io.Copy(io.Discard, server)
+	}()
+
+	w, err := NewConn(client, sess, true, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	var wg sync.WaitGroup
+	for i := 0; i < 8; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for j := 0; j < 32; j++ {
+				_, _ = w.Write([]byte("0123456789abcdef"))
+			}
+		}()
+	}
+	wg.Add(1)
+	go func() {
+		defer wg.Done()
+		_ = w.Close()
+	}()
+	wg.Wait()
+}