diff --git a/ackhandler/sent_packet_handler.go b/ackhandler/sent_packet_handler.go
index e7984723..d3a36de3 100644
--- a/ackhandler/sent_packet_handler.go
+++ b/ackhandler/sent_packet_handler.go
@@ -328,6 +328,11 @@ func (h *sentPacketHandler) GetStopWaitingFrame(force bool) *frames.StopWaitingF
 func (h *sentPacketHandler) SendingAllowed() bool {
 	congestionLimited := h.bytesInFlight > h.congestion.GetCongestionWindow()
 	maxTrackedLimited := protocol.PacketNumber(len(h.retransmissionQueue)+h.packetHistory.Len()) >= protocol.MaxTrackedSentPackets
+	if congestionLimited {
+		utils.Debugf("Congestion limited: bytes in flight %d, window %d",
+			h.bytesInFlight,
+			h.congestion.GetCongestionWindow())
+	}
 	return !(congestionLimited || maxTrackedLimited)
 }
 
@@ -342,7 +347,11 @@ func (h *sentPacketHandler) retransmitOldestTwoPackets() {
 
 func (h *sentPacketHandler) queueRTO(el *PacketElement) {
 	packet := &el.Value
-	utils.Debugf("\tQueueing packet 0x%x for retransmission (RTO)", packet.PacketNumber)
+	utils.Debugf(
+		"\tQueueing packet 0x%x for retransmission (RTO), %d outstanding",
+		packet.PacketNumber,
+		h.packetHistory.Len(),
+	)
 	h.queuePacketForRetransmission(el)
 	h.congestion.OnPacketLost(packet.PacketNumber, packet.Length, h.bytesInFlight)
 	h.congestion.OnRetransmissionTimeout(true)
@@ -352,11 +361,7 @@ func (h *sentPacketHandler) queuePacketForRetransmission(packetElement *PacketEl
 	packet := &packetElement.Value
 	h.bytesInFlight -= packet.Length
 	h.retransmissionQueue = append(h.retransmissionQueue, packet)
-
 	h.packetHistory.Remove(packetElement)
-
-	// strictly speaking, this is only necessary for RTO retransmissions
-	// this is because FastRetransmissions are triggered by missing ranges in ACKs, and then the LargestAcked will already be higher than the packet number of the retransmitted packet
 	h.stopWaitingManager.QueuedRetransmissionForPacketNumber(packet.PacketNumber)
 }
 
diff --git a/session.go b/session.go
index 5b7a96fe..130a55cd 100644
--- a/session.go
+++ b/session.go
@@ -552,10 +552,7 @@ func (s *session) sendPacket() error {
 
 		// get WindowUpdate frames
 		// this call triggers the flow controller to increase the flow control windows, if necessary
-		windowUpdateFrames, err := s.getWindowUpdateFrames()
-		if err != nil {
-			return err
-		}
+		windowUpdateFrames := s.getWindowUpdateFrames()
 		for _, wuf := range windowUpdateFrames {
 			controlFrames = append(controlFrames, wuf)
 		}
@@ -572,7 +569,7 @@ func (s *session) sendPacket() error {
 				utils.Debugf("\tDequeueing handshake retransmission for packet 0x%x", retransmitPacket.PacketNumber)
 				stopWaitingFrame := s.sentPacketHandler.GetStopWaitingFrame(true)
 				var packet *packedPacket
-				packet, err = s.packer.RetransmitNonForwardSecurePacket(stopWaitingFrame, retransmitPacket)
+				packet, err := s.packer.RetransmitNonForwardSecurePacket(stopWaitingFrame, retransmitPacket)
 				if err != nil {
 					return err
 				}
@@ -594,7 +591,7 @@ func (s *session) sendPacket() error {
 						// only retransmit WindowUpdates if the stream is not yet closed and the we haven't sent another WindowUpdate with a higher ByteOffset for the stream
 						var currentOffset protocol.ByteCount
 						f := frame.(*frames.WindowUpdateFrame)
-						currentOffset, err = s.flowControlManager.GetReceiveWindow(f.StreamID)
+						currentOffset, err := s.flowControlManager.GetReceiveWindow(f.StreamID)
 						if err == nil && f.ByteOffset >= currentOffset {
 							controlFrames = append(controlFrames, frame)
 						}
@@ -779,13 +776,13 @@ func (s *session) tryDecryptingQueuedPackets() {
 	s.undecryptablePackets = s.undecryptablePackets[:0]
 }
 
-func (s *session) getWindowUpdateFrames() ([]*frames.WindowUpdateFrame, error) {
+func (s *session) getWindowUpdateFrames() []*frames.WindowUpdateFrame {
 	updates := s.flowControlManager.GetWindowUpdates()
 	res := make([]*frames.WindowUpdateFrame, len(updates))
 	for i, u := range updates {
 		res[i] = &frames.WindowUpdateFrame{StreamID: u.StreamID, ByteOffset: u.Offset}
 	}
-	return res, nil
+	return res
 }
 
 func (s *session) ackAlarmChanged(t time.Time) {
diff --git a/session_test.go b/session_test.go
index f228b12f..e0307b87 100644
--- a/session_test.go
+++ b/session_test.go
@@ -1423,8 +1423,7 @@ var _ = Describe("Session", func() {
 		It("gets stream level window updates", func() {
 			err := sess.flowControlManager.AddBytesRead(1, protocol.ReceiveStreamFlowControlWindow)
 			Expect(err).NotTo(HaveOccurred())
-			frames, err := sess.getWindowUpdateFrames()
-			Expect(err).NotTo(HaveOccurred())
+			frames := sess.getWindowUpdateFrames()
 			Expect(frames).To(HaveLen(1))
 			Expect(frames[0].StreamID).To(Equal(protocol.StreamID(1)))
 			Expect(frames[0].ByteOffset).To(Equal(protocol.ReceiveStreamFlowControlWindow * 2))
@@ -1435,8 +1434,7 @@ var _ = Describe("Session", func() {
 			Expect(err).NotTo(HaveOccurred())
 			err = sess.flowControlManager.AddBytesRead(5, protocol.ReceiveConnectionFlowControlWindow)
 			Expect(err).NotTo(HaveOccurred())
-			frames, err := sess.getWindowUpdateFrames()
-			Expect(err).NotTo(HaveOccurred())
+			frames := sess.getWindowUpdateFrames()
 			Expect(frames).To(HaveLen(1))
 			Expect(frames[0].StreamID).To(Equal(protocol.StreamID(0)))
 			Expect(frames[0].ByteOffset).To(Equal(protocol.ReceiveConnectionFlowControlWindow * 2))