diff --git a/prober/derp.go b/prober/derp.go
index e21c8ce76..c7a82317d 100644
--- a/prober/derp.go
+++ b/prober/derp.go
@@ -425,6 +425,24 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 	txRecords := make([]txRecord, 0, packetsPerSecond*int(packetTimeout.Seconds()))
 	var txRecordsMu sync.Mutex
 
+	// applyTimeouts walks over txRecords and expires any records that are older
+	// than packetTimeout, recording in metrics that they were removed.
+	applyTimeouts := func() {
+		txRecordsMu.Lock()
+		defer txRecordsMu.Unlock()
+
+		now := time.Now()
+		recs := txRecords[:0]
+		for _, r := range txRecords {
+			if now.Sub(r.at) > packetTimeout {
+				packetsDropped.Add(1)
+			} else {
+				recs = append(recs, r)
+			}
+		}
+		txRecords = recs
+	}
+
 	// Send the packets.
 	sendErrC := make(chan error, 1)
 	// TODO: construct a disco CallMeMaybe in the same fashion as magicsock, e.g. magic bytes, src pub, seal payload.
@@ -445,10 +463,12 @@ func runDerpProbeQueuingDelayContinously(ctx context.Context, from, to *tailcfg.
 			case <-ctx.Done():
 				return
 			case <-t.C:
+				applyTimeouts()
 				txRecordsMu.Lock()
 				if len(txRecords) == cap(txRecords) {
 					txRecords = slices.Delete(txRecords, 0, 1)
 					packetsDropped.Add(1)
+					log.Printf("unexpected: overflow in txRecords")
 				}
 				txRecords = append(txRecords, txRecord{time.Now(), seq})
 				txRecordsMu.Unlock()