diff --git a/compiler/parser/api.go b/compiler/parser/api.go
index be73b641ed..114f375cc9 100644
--- a/compiler/parser/api.go
+++ b/compiler/parser/api.go
@@ -99,20 +99,22 @@ func (e *Error) Error() string {
 	if e.sset == nil {
 		return e.Msg
 	}
-	src := e.sset.SourceOf(e.Pos)
-	start := src.Position(e.Pos)
-	end := src.Position(e.End)
 	var b strings.Builder
 	b.WriteString(e.Msg)
-	if src.Filename != "" {
-		fmt.Fprintf(&b, " in %s", src.Filename)
-	}
-	line := src.LineOfPos(e.sset.Text, e.Pos)
-	fmt.Fprintf(&b, " at line %d, column %d:\n%s\n", start.Line, start.Column, line)
-	if end.IsValid() {
-		formatSpanError(&b, line, start, end)
-	} else {
-		formatPointError(&b, start)
+	if e.Pos >= 0 {
+		src := e.sset.SourceOf(e.Pos)
+		start := src.Position(e.Pos)
+		end := src.Position(e.End)
+		if src.Filename != "" {
+			fmt.Fprintf(&b, " in %s", src.Filename)
+		}
+		line := src.LineOfPos(e.sset.Text, e.Pos)
+		fmt.Fprintf(&b, " at line %d, column %d:\n%s\n", start.Line, start.Column, line)
+		if end.IsValid() {
+			formatSpanError(&b, line, start, end)
+		} else {
+			formatPointError(&b, start)
+		}
 	}
 	return b.String()
 }
diff --git a/compiler/semantic/analyzer.go b/compiler/semantic/analyzer.go
index 6270555117..06c883d108 100644
--- a/compiler/semantic/analyzer.go
+++ b/compiler/semantic/analyzer.go
@@ -18,7 +18,7 @@ import (
 func Analyze(ctx context.Context, seq ast.Seq, source *data.Source, head *lakeparse.Commitish) (dag.Seq, error) {
 	a := newAnalyzer(ctx, source, head)
 	s := a.semSeq(seq)
-	s = a.checkOutputs(true, s)
+	s, _ = a.checkSeqOutputs(true, s)
 	if a.errors != nil {
 		return nil, a.errors
 	}
@@ -30,7 +30,7 @@ func Analyze(ctx context.Context, seq ast.Seq, source *data.Source, head *lakepa
 func AnalyzeAddSource(ctx context.Context, seq ast.Seq, source *data.Source, head *lakeparse.Commitish) (dag.Seq, error) {
 	a := newAnalyzer(ctx, source, head)
 	s := a.semSeq(seq)
-	s = a.checkOutputs(true, s)
+	s, _ = a.checkSeqOutputs(true, s)
 	if a.errors != nil {
 		return nil, a.errors
 	}
@@ -151,54 +151,76 @@ func badOp() dag.Op {
 }
 
 func (a *analyzer) error(n ast.Node, err error) {
-	a.errors.Append(err.Error(), n.Pos(), n.End())
+	pos, end := -1, -1
+	if n != nil {
+		pos, end = n.Pos(), n.End()
+	}
+	a.errors.Append(err.Error(), pos, end)
 }
 
-func (a *analyzer) checkOutputs(isLeaf bool, seq dag.Seq) dag.Seq {
+func (a *analyzer) checkSeqOutputs(isLeaf bool, seq dag.Seq) (dag.Seq, bool) {
 	if len(seq) == 0 {
-		return seq
+		return seq, false
 	}
 	// - Report an error in any outputs are not located in the leaves.
 	// - Add output operators to any leaves where they do not exist.
-	lastN := len(seq) - 1
+	last := len(seq) - 1
+	var blocked bool
 	for i, o := range seq {
-		isLast := lastN == i
-		switch o := o.(type) {
-		case *dag.Output:
-			if !isLast || !isLeaf {
-				n, ok := a.outputs[o]
-				if !ok {
-					panic("system error: untracked user output")
-				}
-				a.error(n, errors.New("output operator must be at flowgraph leaf"))
-			}
-		case *dag.Scope:
-			o.Body = a.checkOutputs(isLast && isLeaf, o.Body)
-		case *dag.Scatter:
-			for k := range o.Paths {
-				o.Paths[k] = a.checkOutputs(isLast && isLeaf, o.Paths[k])
-			}
-		case *dag.Over:
-			o.Body = a.checkOutputs(false, o.Body)
-		case *dag.Fork:
-			for k := range o.Paths {
-				o.Paths[k] = a.checkOutputs(isLast && isLeaf, o.Paths[k])
-			}
-		case *dag.Switch:
-			for k := range o.Cases {
-				o.Cases[k].Path = a.checkOutputs(isLast && isLeaf, o.Cases[k].Path)
-			}
-		case *dag.Mirror:
-			o.Main = a.checkOutputs(isLast && isLeaf, o.Main)
-			o.Mirror = a.checkOutputs(isLast && isLeaf, o.Mirror)
+		if blocked {
+			// XXX We need to map all DAG ops to their AST equivalent so we can
+			// get location specific errors.
+			a.error(nil, errors.New("unreachable operator"))
+			return seq, true
 		}
+		blocked = a.checkOpOutput(o, isLeaf, last == i)
 	}
-	switch seq[lastN].(type) {
-	case *dag.Scope, *dag.Output, *dag.Scatter, *dag.Fork, *dag.Switch, *dag.Mirror:
+	switch seq[last].(type) {
+	case *dag.Scope, *dag.Output, *dag.Scatter, *dag.Fork, *dag.Switch, *dag.Mirror, *dag.Over:
 	default:
 		if isLeaf {
-			return append(seq, &dag.Output{Kind: "Output", Name: "main"})
+			seq.Append(&dag.Output{Kind: "Output", Name: "main"})
+			blocked = true
+		}
+	}
+	return seq, blocked
+}
+
+func (a *analyzer) checkOpOutput(o dag.Op, isLeafSeq bool, isLast bool) bool {
+	var blocked bool
+	switch o := o.(type) {
+	case *dag.Output:
+		blocked = true
+	case *dag.Scope:
+		o.Body, blocked = a.checkSeqOutputs(isLast && isLeafSeq, o.Body)
+	case *dag.Scatter:
+		blocked = true
+		for k := range o.Paths {
+			var pathBlocked bool
+			o.Paths[k], pathBlocked = a.checkSeqOutputs(isLast && isLeafSeq, o.Paths[k])
+			blocked = blocked && pathBlocked
+		}
+	case *dag.Over:
+		o.Body, blocked = a.checkSeqOutputs(isLast && isLeafSeq, o.Body)
+	case *dag.Fork:
+		blocked = true
+		for k := range o.Paths {
+			var pathBlocked bool
+			o.Paths[k], pathBlocked = a.checkSeqOutputs(isLast && isLeafSeq, o.Paths[k])
+			blocked = blocked && pathBlocked
+		}
+	case *dag.Switch:
+		blocked = true
+		for k := range o.Cases {
+			var pathBlocked bool
+			o.Cases[k].Path, pathBlocked = a.checkSeqOutputs(isLast && isLeafSeq, o.Cases[k].Path)
+			blocked = blocked && pathBlocked
 		}
+	case *dag.Mirror:
+		var mainOut, mirrorOut bool
+		o.Main, mainOut = a.checkSeqOutputs(isLast && isLeafSeq, o.Main)
+		o.Mirror, mirrorOut = a.checkSeqOutputs(isLast && isLeafSeq, o.Mirror)
+		blocked = mainOut && mirrorOut
 	}
-	return seq
+	return blocked
 }
diff --git a/zfmt/ztests/output.yaml b/zfmt/ztests/output.yaml
index be4d0b1ecd..375197f80c 100644
--- a/zfmt/ztests/output.yaml
+++ b/zfmt/ztests/output.yaml
@@ -1,7 +1,14 @@
 script: |
   zc -C -s 'fork (=> output foo => pass)'
   echo '// ==='
-  zc -C -s 'switch x (case "foo" => output foo case "bar" => pass)'
+  zc -C -s 'switch x (case "foo" => output foo case "bar" => pass) | sort this'
+  echo '// ==='
+  zc -C -s 'over this => ( sum(this) | output bar )'
+  # error cases
+  ! zc -C -s 'over this => ( sum(this) | output bar ) | x := "foo"'
+  ! zc -C -s 'fork (=> output foo | "yield foo" => pass) | sort this'
+  ! zc -C -s 'fork (=> output foo => output bar) | count()'
+  ! zc -C -s 'switch x (case "foo" => output foo case "bar" => output bar) | count()'
 
 outputs:
   - name: stdout
@@ -21,5 +28,20 @@ outputs:
             output foo
           case "bar" =>
             pass
-            | output main
         )
+        | sort this
+        | output main
+      // ===
+      reader
+      | over this => (
+        summarize
+            sum:=sum(this)
+        | yield sum
+        | output bar
+      )
+  - name: stderr
+    data: |
+      unreachable operator
+      unreachable operator
+      unreachable operator
+      unreachable operator