diff --git a/cmd/qrunproxy/mockshow.go b/cmd/qrunproxy/mockshow.go
index 81ff2b7..d96d200 100644
--- a/cmd/qrunproxy/mockshow.go
+++ b/cmd/qrunproxy/mockshow.go
@@ -80,11 +80,22 @@ func GenerateMockShow(numTracks, numScenes, avgCuesPerScene, avgBlocksPerCue int
 
 	chainFromByTrack := make(map[int]*Block)
 	needsEnd := make(map[string]*Block)
+	triggerIdx := make(map[TriggerSource]*Trigger)
 	allowedTracks := make(map[int]bool, numTracks)
 	for i := range numTracks {
 		allowedTracks[i] = true
 	}
 
+	addTrigger := func(source TriggerSource, target TriggerTarget) {
+		if t := triggerIdx[source]; t != nil {
+			t.Targets = append(t.Targets, target)
+			return
+		}
+		t := &Trigger{Source: source, Targets: []TriggerTarget{target}}
+		show.Triggers = append(show.Triggers, t)
+		triggerIdx[source] = t
+	}
+
 	for scene := 1; scene <= numScenes; scene++ {
 		cuesInScene := 1 + rng.IntN(avgCuesPerScene*2)
 
@@ -126,11 +137,27 @@ func GenerateMockShow(numTracks, numScenes, avgCuesPerScene, avgBlocksPerCue int
 				block := randBlock(trackIdx)
 				show.Blocks = append(show.Blocks, block)
 				if prev := chainFromByTrack[trackIdx]; prev != nil {
-					show.Triggers = append(show.Triggers, &Trigger{
-						Source:  TriggerSource{Block: prev.ID, Signal: "END"},
-						Targets: []TriggerTarget{{Block: block.ID, Hook: "START"}},
-					})
+					addTrigger(
+						TriggerSource{Block: prev.ID, Signal: "END"},
+						TriggerTarget{Block: block.ID, Hook: "START"},
+					)
 					delete(needsEnd, prev.ID)
+				} else if rng.Float64() < 0.3 {
+					var crossSrc *Block
+					for ti, blk := range chainFromByTrack {
+						if ti != trackIdx && blk != nil && needsEnd[blk.ID] == nil {
+							crossSrc = blk
+							break
+						}
+					}
+					if crossSrc != nil {
+						addTrigger(
+							TriggerSource{Block: crossSrc.ID, Signal: "END"},
+							TriggerTarget{Block: block.ID, Hook: "START"},
+						)
+					} else {
+						cueTargets = append(cueTargets, TriggerTarget{Block: block.ID, Hook: "START"})
+					}
 				} else {
 					cueTargets = append(cueTargets, TriggerTarget{Block: block.ID, Hook: "START"})
 				}
diff --git a/cmd/qrunproxy/show.go b/cmd/qrunproxy/show.go
index ccce6ee..29924e9 100644
--- a/cmd/qrunproxy/show.go
+++ b/cmd/qrunproxy/show.go
@@ -26,6 +26,14 @@ type Trigger struct {
 	Targets []TriggerTarget `json:"targets"`
 }
 
+func (t *Trigger) String() string {
+	s := fmt.Sprintf("%s/%s ->", t.Source.Block, t.Source.Signal)
+	for _, target := range t.Targets {
+		s += fmt.Sprintf(" %s/%s", target.Block, target.Hook)
+	}
+	return s
+}
+
 type TriggerSource struct {
 	Block  string `json:"block"`
 	Signal string `json:"signal"`
@@ -92,11 +100,33 @@ func (show *Show) Validate() error {
 	hookTargeted := map[blockEvent]bool{}
 	startTargeted := map[string]bool{}
 	sourceUsed := map[blockEvent]bool{}
+	signalTargetedBy := map[blockEvent]*Trigger{}
+
+	for _, trigger := range show.Triggers {
+		for _, target := range trigger.Targets {
+			signalTargetedBy[blockEvent{target.Block, target.Hook}] = trigger
+		}
+	}
+
 	for _, trigger := range show.Triggers {
 		sourceBlock := blocksByID[trigger.Source.Block]
 		if sourceBlock == nil {
 			return fmt.Errorf("trigger source block %q not found", trigger.Source.Block)
 		}
+
+		targetedTracks := map[string]string{}
+		for _, target := range trigger.Targets {
+			targetBlock := blocksByID[target.Block]
+			if prev, ok := targetedTracks[targetBlock.Track]; ok {
+				return fmt.Errorf("trigger conflict: %s targets multiple blocks on track %q (%q and %q)",
+					trigger, targetBlock.Track, prev, target.Block)
+			}
+			targetedTracks[targetBlock.Track] = target.Block
+		}
+
+		if t, ok := signalTargetedBy[blockEvent{trigger.Source.Block, trigger.Source.Signal}]; ok {
+			return fmt.Errorf("trigger conflict: %s vs %s", t, trigger)
+		}
 		if !isValidEventForBlock(sourceBlock, trigger.Source.Signal) {
 			return fmt.Errorf("trigger source signal %q is invalid for block %q", trigger.Source.Signal, trigger.Source.Block)
 		}
diff --git a/cmd/qrunproxy/timeline.go b/cmd/qrunproxy/timeline.go
index 2d706aa..6a58291 100644
--- a/cmd/qrunproxy/timeline.go
+++ b/cmd/qrunproxy/timeline.go
@@ -257,13 +257,16 @@ func (tl *Timeline) buildConstraints() {
 }
 
 func (tl *Timeline) assignRows() error {
-	for range 1000000 {
+	for i := range 1000000 {
 		if tl.enforceConstraints() {
 			continue
 		}
 		if tl.enforceExclusives() {
 			continue
 		}
+		if i > 0 {
+			fmt.Printf("assignRows: converged in %d iterations\n", i)
+		}
 		return nil
 	}
 	for _, c := range tl.constraints {
@@ -284,6 +287,7 @@ func (tl *Timeline) enforceConstraints() bool {
 		if c.satisfied() {
 			continue
 		}
+		fmt.Printf("enforceConstraints: unsatisfied %s\n", c)
 		switch c.kind {
 		case constraintSameRow:
 			if c.a.row < c.b.row {
@@ -304,6 +308,7 @@ func (tl *Timeline) enforceExclusives() bool {
 		if g.satisfied(tl.Tracks) {
 			continue
 		}
+		fmt.Printf("enforceExclusives: unsatisfied %s\n", g)
 		row := g.members[0].row
 		memberTracks := map[*TimelineTrack]bool{}
 		for _, m := range g.members {
@@ -316,6 +321,7 @@ func (tl *Timeline) enforceExclusives() bool {
 			if !t.cellTypeAt(row, CellEvent, CellTitle, CellSignal) {
 				continue
 			}
+			fmt.Printf("enforceExclusives: inserting gap in track %s at row %d\n", t.ID, row)
 			tl.insertGap(t, row)
 			return true
 		}
@@ -344,6 +350,7 @@ func (tl *Timeline) isAllRemovableGapRow(row int, except *TimelineTrack) bool {
 }
 
 func (tl *Timeline) removeGapAt(track *TimelineTrack, index int) {
+	fmt.Printf("removeGapAt: track %s row %d\n", track.ID, index)
 	track.Cells = append(track.Cells[:index], track.Cells[index+1:]...)
 	tl.reindexRowsFrom(track, index)
 }
@@ -355,8 +362,9 @@ func (tl *Timeline) reindexRowsFrom(track *TimelineTrack, start int) {
 }
 
 func (tl *Timeline) insertGap(track *TimelineTrack, beforeIndex int) {
-
+	fmt.Printf("insertGap: track %s before %d\n", track.ID, beforeIndex)
 	if tl.isAllRemovableGapRow(beforeIndex, track) {
+		fmt.Printf("insertGap: found removable gap row at %d\n", beforeIndex)
 		for _, t := range tl.Tracks {
 			if t == track {
 				continue