diff --git a/consensus/reactor.go b/consensus/reactor.go
index 9b515ac44..22c4182c0 100644
--- a/consensus/reactor.go
+++ b/consensus/reactor.go
@@ -168,7 +168,7 @@ func (conR *ConsensusReactor) Receive(chId byte, peer *p2p.Peer, msgBytes []byte
 			ps.ApplyProposalPOLMessage(msg)
 		case *BlockPartMessage:
 			ps.SetHasProposalBlockPart(msg.Height, msg.Round, msg.Part.Proof.Index)
-			_, err = conR.conS.AddProposalBlockPart(msg.Height, msg.Round, msg.Part)
+			_, err = conR.conS.AddProposalBlockPart(msg.Height, msg.Part)
 		default:
 			log.Warn(Fmt("Unknown message type %v", reflect.TypeOf(msg)))
 		}
@@ -342,12 +342,12 @@ OUTER_LOOP:
 			if index, ok := rs.ProposalBlockParts.BitArray().Sub(prs.ProposalBlockParts.Copy()).PickRandom(); ok {
 				part := rs.ProposalBlockParts.GetPart(index)
 				msg := &BlockPartMessage{
-					Height: rs.Height,
-					Round:  rs.Round,
+					Height: rs.Height, // This tells peer that this part applies to us.
+					Round:  rs.Round,  // This tells peer that this part applies to us.
 					Part:   part,
 				}
 				peer.Send(DataChannel, msg)
-				ps.SetHasProposalBlockPart(rs.Height, rs.Round, index)
+				ps.SetHasProposalBlockPart(prs.Height, prs.Round, index)
 				continue OUTER_LOOP
 			}
 		}
@@ -374,8 +374,8 @@ OUTER_LOOP:
 				}
 				// Send the part
 				msg := &BlockPartMessage{
-					Height: prs.Height,
-					Round:  prs.Round,
+					Height: prs.Height, // Not our height, so it doesn't matter.
+					Round:  prs.Round,  // Not our height, so it doesn't matter.
 					Part:   part,
 				}
 				peer.Send(DataChannel, msg)
@@ -389,13 +389,16 @@ OUTER_LOOP:
 		}
 
 		// If height and round don't match, sleep.
-		if rs.Height != prs.Height || rs.Round != prs.Round {
+		if (rs.Height != prs.Height) || (rs.Round != prs.Round) {
 			//log.Debug("Peer Height|Round mismatch, sleeping", "peerHeight", prs.Height, "peerRound", prs.Round, "peer", peer)
 			time.Sleep(peerGossipSleepDuration)
 			continue OUTER_LOOP
 		}
 
 		// By here, height and round match.
+		// Proposal block parts were already matched and sent if any were wanted.
+		// (These can match on hash so the round doesn't matter)
+		// Now consider sending other things, like the Proposal itself.
 
 		// Send Proposal && ProposalPOL BitArray?
 		if rs.Proposal != nil && !prs.Proposal {
@@ -406,8 +409,9 @@ OUTER_LOOP:
 				ps.SetHasProposal(rs.Proposal)
 			}
 			// ProposalPOL.
-			// Must be in the same channel, sequential.
-			// That is, peer must receive ProposalMessage first.
+			// Peer must receive ProposalMessage first.
+			// rs.Proposal was validated, so rs.Proposal.POLRound <= rs.Round,
+			// so we definitely have rs.Votes.Prevotes(rs.Proposal.POLRound).
 			if 0 <= rs.Proposal.POLRound {
 				msg := &ProposalPOLMessage{
 					Height:           rs.Height,
@@ -530,11 +534,13 @@ OUTER_LOOP:
 		// If peer is lagging by height 1, send LastCommit.
 		if prs.Height != 0 && prs.Height == rs.Height-1 {
 			if prs.Round == rs.LastCommit.Round() {
+				// NOTE: We prefer to use prs.Precommits if
+				// prs.Round matches prs.CatchupCommitRound.
 				if trySendVote(rs.LastCommit, &prs.Precommits) {
 					continue OUTER_LOOP
 				}
 			} else {
-				ps.SetCatchupCommitRound(prs.Height, rs.LastCommit.Round())
+				ps.EnsureCatchupCommitRound(prs.Height, rs.LastCommit.Round())
 				if trySendVote(rs.LastCommit, &prs.CatchupCommit) {
 					continue OUTER_LOOP
 				}
@@ -548,15 +554,8 @@ OUTER_LOOP:
 			// which contains precommit signatures for prs.Height.
 			validation := conR.blockStore.LoadBlockValidation(prs.Height)
 			log.Debug("Loaded BlockValidation for catch-up", "height", prs.Height, "validation", validation)
-			// Peer's CatchupCommitRound should be -1 or equal to the validation's precommit rounds.
-			// If not, warn.
-			if prs.CatchupCommitRound == -1 {
-				ps.SetCatchupCommitRound(prs.Height, validation.Round())
-				continue OUTER_LOOP // Get prs := ps.GetRoundState() again.
-			} else if prs.CatchupCommitRound != int(validation.Round()) {
-				log.Warn("Peer's CatchupCommitRound during catchup not equal to commit round",
-					"height", prs.Height, "validation", validation, "prs.CatchupCommitRound", prs.CatchupCommitRound)
-			} else if trySendPrecommitFromValidation(validation, &prs.CatchupCommit) {
+			ps.EnsureCatchupCommitRound(prs.Height, validation.Round())
+			if trySendPrecommitFromValidation(validation, &prs.CatchupCommit) {
 				continue OUTER_LOOP
 			}
 		}
@@ -656,6 +655,8 @@ func (ps *PeerState) SetHasProposalBlockPart(height uint, round uint, index uint
 }
 
 // prs: If given, will also update this PeerRoundState copy.
+// NOTE: It's important to make sure that numValidators actually matches
+// what the node sees as the number of validators for height.
 func (ps *PeerState) EnsureVoteBitArrays(height uint, numValidators uint, prs *PeerRoundState) {
 	ps.mtx.Lock()
 	defer ps.mtx.Unlock()
@@ -683,9 +684,9 @@ func (ps *PeerState) EnsureVoteBitArrays(height uint, numValidators uint, prs *P
 	if prs != nil {
 		prs.Prevotes = ps.Prevotes
 		prs.Precommits = ps.Precommits
-		prs.LastCommit = ps.LastCommit
 		prs.CatchupCommit = ps.CatchupCommit
 		prs.ProposalPOL = ps.ProposalPOL
+		prs.LastCommit = ps.LastCommit
 	}
 }
 
@@ -726,7 +727,8 @@ func (ps *PeerState) setHasVote(height uint, round uint, type_ byte, index uint)
 	}
 }
 
-func (ps *PeerState) SetCatchupCommitRound(height, round uint) {
+// NOTE: 'round' is what we know to be the commit round for height.
+func (ps *PeerState) EnsureCatchupCommitRound(height, round uint) {
 	ps.mtx.Lock()
 	defer ps.mtx.Unlock()
 
@@ -734,12 +736,10 @@ func (ps *PeerState) SetCatchupCommitRound(height, round uint) {
 		return
 	}
 	if ps.CatchupCommitRound != -1 && ps.CatchupCommitRound != int(round) {
-		log.Warn("Conflicting CatchupCommitRound",
-			"height", height,
-			"orig", ps.CatchupCommitRound,
-			"new", round,
-		)
-		// TODO think harder
+		panic(Fmt("Conflicting CatchupCommitRound. Height: %v, Orig: %v, New: %v", height, ps.CatchupCommitRound, round))
+	}
+	if ps.CatchupCommitRound == int(round) {
+		return // Nothing to do!
 	}
 	ps.CatchupCommitRound = int(round)
 	ps.CatchupCommit = nil
@@ -773,6 +773,9 @@ func (ps *PeerState) ApplyNewRoundStepMessage(msg *NewRoundStepMessage, rs *Roun
 	}
 	if psHeight == msg.Height && psRound != msg.Round && int(msg.Round) == psCatchupCommitRound {
 		// Peer caught up to CatchupCommitRound.
+		// Preserve psCatchupCommit!
+		// NOTE: We prefer to use prs.Precommits if
+		// pr.Round matches pr.CatchupCommitRound.
 		ps.Precommits = psCatchupCommit
 	}
 	if psHeight != msg.Height {
diff --git a/consensus/state.go b/consensus/state.go
index cd22ac233..1811c1175 100644
--- a/consensus/state.go
+++ b/consensus/state.go
@@ -499,7 +499,7 @@ func (cs *ConsensusState) EnterPropose(height uint, round uint) {
 
 		// If we already have the proposal + POL, then goto Prevote
 		if cs.isProposalComplete() {
-			go cs.EnterPrevote(height, round)
+			go cs.EnterPrevote(height, cs.Round)
 		}
 	}()
 
@@ -965,7 +965,7 @@ func (cs *ConsensusState) SetProposal(proposal *Proposal) error {
 }
 
 // NOTE: block is not necessarily valid.
-func (cs *ConsensusState) AddProposalBlockPart(height uint, round uint, part *types.Part) (added bool, err error) {
+func (cs *ConsensusState) AddProposalBlockPart(height uint, part *types.Part) (added bool, err error) {
 	cs.mtx.Lock()
 	defer cs.mtx.Unlock()
 
@@ -991,7 +991,7 @@ func (cs *ConsensusState) AddProposalBlockPart(height uint, round uint, part *ty
 		log.Debug("Received complete proposal", "hash", cs.ProposalBlock.Hash())
 		if cs.Step == RoundStepPropose && cs.isProposalComplete() {
 			// Move onto the next step
-			go cs.EnterPrevote(height, round)
+			go cs.EnterPrevote(height, cs.Round)
 		} else if cs.Step == RoundStepCommit {
 			// If we're waiting on the proposal block...
 			cs.tryFinalizeCommit(height)