diff --git a/ssl/test/runner/common.go b/ssl/test/runner/common.go
index 99b0c04..3e37f1d 100644
--- a/ssl/test/runner/common.go
+++ b/ssl/test/runner/common.go
@@ -610,6 +610,12 @@
 
 	// PacketAdaptor is the packetAdaptor to use to simulate timeouts.
 	PacketAdaptor *packetAdaptor
+
+	// ReorderHandshakeFragments, if true, causes handshake fragments in
+	// DTLS to overlap and be sent in the wrong order. It also causes
+	// pre-CCS flights to be sent twice. (Post-CCS flights consist of
+	// Finished and will trigger a spurious retransmit.)
+	ReorderHandshakeFragments bool
 }
 
 func (c *Config) serverInit() {
diff --git a/ssl/test/runner/conn.go b/ssl/test/runner/conn.go
index 307f61d..86a7b7d 100644
--- a/ssl/test/runner/conn.go
+++ b/ssl/test/runner/conn.go
@@ -69,8 +69,9 @@
 	// DTLS state
 	sendHandshakeSeq uint16
 	recvHandshakeSeq uint16
-	handMsg          []byte // pending assembled handshake message
-	handMsgLen       int    // handshake message length, not including the header
+	handMsg          []byte   // pending assembled handshake message
+	handMsgLen       int      // handshake message length, not including the header
+	pendingFragments [][]byte // pending outgoing handshake fragments.
 
 	tmp [16]byte
 }
diff --git a/ssl/test/runner/dtls.go b/ssl/test/runner/dtls.go
index 8e4b8a6..f84530f 100644
--- a/ssl/test/runner/dtls.go
+++ b/ssl/test/runner/dtls.go
@@ -20,6 +20,7 @@
 	"errors"
 	"fmt"
 	"io"
+	"math/rand"
 	"net"
 )
 
@@ -125,10 +126,6 @@
 	// FragmentAcrossChangeCipherSpec. (Which is unfortunate
 	// because OpenSSL's DTLS implementation will probably accept
 	// such fragmentation and could do with a fix + tests.)
-	if len(data) < 4 {
-		// This should not happen.
-		panic(data)
-	}
 	header := data[:4]
 	data = data[4:]
 
@@ -151,12 +148,13 @@
 		fragment = append(fragment, byte(m>>16), byte(m>>8), byte(m))
 		fragment = append(fragment, data[:m]...)
 
-		// TODO(davidben): A real DTLS implementation needs to
-		// retransmit handshake messages. For testing purposes, we don't
-		// actually care.
-		_, err = c.dtlsWriteRawRecord(recordTypeHandshake, fragment)
-		if err != nil {
-			break
+		// Buffer the fragment for later. They will be sent (and
+		// reordered) on flush.
+		c.pendingFragments = append(c.pendingFragments, fragment)
+
+		if c.config.Bugs.ReorderHandshakeFragments && m > (maxLen+1)/2 {
+			// Overlap each fragment by half.
+			m = (maxLen + 1) / 2
 		}
 		n += m
 		data = data[m:]
@@ -168,6 +166,38 @@
 	return
 }
 
+func (c *Conn) dtlsFlushHandshake(duplicate bool) error {
+	if !c.isDTLS {
+		return nil
+	}
+
+	var fragments []byte
+	fragments, c.pendingFragments = c.pendingFragments, fragments
+
+	if c.config.Bugs.ReorderHandshakeFragments {
+		if duplicate {
+			fragments = append(fragments, fragments...)
+		}
+		perm := rand.New(rand.NewSource(0)).Perm(len(fragments))
+		tmp := make([][]byte, len(fragments))
+		for i := range tmp {
+			tmp[i] = fragments[perm[i]]
+		}
+		fragments = tmp
+	}
+
+	// Send them all.
+	for _, fragment := range fragments {
+		// TODO(davidben): A real DTLS implementation needs to
+		// retransmit handshake messages. For testing purposes, we don't
+		// actually care.
+		if _, err := c.dtlsWriteRawRecord(recordTypeHandshake, fragment); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
 func (c *Conn) dtlsWriteRawRecord(typ recordType, data []byte) (n int, err error) {
 	recordHeaderLen := dtlsRecordHeaderLen
 	maxLen := c.config.Bugs.MaxHandshakeRecordLength
diff --git a/ssl/test/runner/handshake_client.go b/ssl/test/runner/handshake_client.go
index 72a82f8..5ad3602 100644
--- a/ssl/test/runner/handshake_client.go
+++ b/ssl/test/runner/handshake_client.go
@@ -214,6 +214,9 @@
 		helloBytes = hello.marshal()
 		c.writeRecord(recordTypeHandshake, helloBytes)
 	}
+	if err := c.dtlsFlushHandshake(true); err != nil {
+		return err
+	}
 
 	if err := c.simulatePacketLoss(nil); err != nil {
 		return err
@@ -237,6 +240,9 @@
 			hello.cookie = helloVerifyRequest.cookie
 			helloBytes = hello.marshal()
 			c.writeRecord(recordTypeHandshake, helloBytes)
+			if err := c.dtlsFlushHandshake(true); err != nil {
+				return err
+			}
 
 			if err := c.simulatePacketLoss(nil); err != nil {
 				return err
@@ -327,7 +333,10 @@
 		// Most retransmits are triggered by a timeout, but the final
 		// leg of the handshake is retransmited upon re-receiving a
 		// Finished.
-		if err := c.simulatePacketLoss(func() { c.writeRecord(recordTypeHandshake, hs.finishedBytes) }); err != nil {
+		if err := c.simulatePacketLoss(func() {
+			c.writeRecord(recordTypeHandshake, hs.finishedBytes)
+			c.dtlsFlushHandshake(false)
+		}); err != nil {
 			return err
 		}
 		if err := hs.readSessionTicket(); err != nil {
@@ -612,6 +621,9 @@
 		hs.writeClientHash(certVerify.marshal())
 		c.writeRecord(recordTypeHandshake, certVerify.marshal())
 	}
+	if err := c.dtlsFlushHandshake(true); err != nil {
+		return err
+	}
 
 	hs.finishedHash.discardHandshakeBuffer()
 
@@ -847,6 +859,9 @@
 		c.writeRecord(recordTypeHandshake, postCCSBytes[:5])
 		postCCSBytes = postCCSBytes[5:]
 	}
+	if err := c.dtlsFlushHandshake(true); err != nil {
+		return err
+	}
 
 	if !c.config.Bugs.SkipChangeCipherSpec &&
 		c.config.Bugs.EarlyChangeCipherSpec == 0 {
@@ -858,6 +873,9 @@
 	}
 
 	c.writeRecord(recordTypeHandshake, postCCSBytes)
+	if err := c.dtlsFlushHandshake(false); err != nil {
+		return err
+	}
 	return nil
 }
 
diff --git a/ssl/test/runner/handshake_server.go b/ssl/test/runner/handshake_server.go
index af521fc..c13de9b 100644
--- a/ssl/test/runner/handshake_server.go
+++ b/ssl/test/runner/handshake_server.go
@@ -75,7 +75,10 @@
 		// Most retransmits are triggered by a timeout, but the final
 		// leg of the handshake is retransmited upon re-receiving a
 		// Finished.
-		if err := c.simulatePacketLoss(func() { c.writeRecord(recordTypeHandshake, hs.finishedBytes) }); err != nil {
+		if err := c.simulatePacketLoss(func() {
+			c.writeRecord(recordTypeHandshake, hs.finishedBytes)
+			c.dtlsFlushHandshake(false)
+		}); err != nil {
 			return err
 		}
 		if err := hs.readFinished(isResume); err != nil {
@@ -146,6 +149,9 @@
 			return false, errors.New("dtls: short read from Rand: " + err.Error())
 		}
 		c.writeRecord(recordTypeHandshake, helloVerifyRequest.marshal())
+		if err := c.dtlsFlushHandshake(true); err != nil {
+			return false, err
+		}
 
 		if err := c.simulatePacketLoss(nil); err != nil {
 			return false, err
@@ -543,6 +549,9 @@
 	helloDone := new(serverHelloDoneMsg)
 	hs.writeServerHash(helloDone.marshal())
 	c.writeRecord(recordTypeHandshake, helloDone.marshal())
+	if err := c.dtlsFlushHandshake(true); err != nil {
+		return err
+	}
 
 	var pub crypto.PublicKey // public key for client auth, if any
 
@@ -836,6 +845,9 @@
 		c.writeRecord(recordTypeHandshake, postCCSBytes[:5])
 		postCCSBytes = postCCSBytes[5:]
 	}
+	if err := c.dtlsFlushHandshake(true); err != nil {
+		return err
+	}
 
 	if !c.config.Bugs.SkipChangeCipherSpec {
 		c.writeRecord(recordTypeChangeCipherSpec, []byte{1})
@@ -846,6 +858,9 @@
 	}
 
 	c.writeRecord(recordTypeHandshake, postCCSBytes)
+	if err := c.dtlsFlushHandshake(false); err != nil {
+		return err
+	}
 
 	c.cipherSuite = hs.suite.id
 
diff --git a/ssl/test/runner/runner.go b/ssl/test/runner/runner.go
index 7fda082..b97bc42 100644
--- a/ssl/test/runner/runner.go
+++ b/ssl/test/runner/runner.go
@@ -652,6 +652,36 @@
 			},
 		},
 	},
+	{
+		protocol: dtls,
+		name:     "ReorderHandshakeFragments-Small-DTLS",
+		config: Config{
+			Bugs: ProtocolBugs{
+				ReorderHandshakeFragments: true,
+				// Small enough that every handshake message is
+				// fragmented.
+				MaxHandshakeRecordLength: 2,
+			},
+		},
+	},
+	{
+		protocol: dtls,
+		name:     "ReorderHandshakeFragments-Large-DTLS",
+		config: Config{
+			Bugs: ProtocolBugs{
+				ReorderHandshakeFragments: true,
+				// Large enough that no handshake message is
+				// fragmented.
+				//
+				// TODO(davidben): Also test interaction of
+				// complete handshake messages with
+				// fragments. The current logic is full of bugs
+				// here, so the reassembly logic needs a rewrite
+				// before those tests will pass.
+				MaxHandshakeRecordLength: 2048,
+			},
+		},
+	},
 }
 
 func doExchange(test *testCase, config *Config, conn net.Conn, messageLen int, isResume bool) error {