diff --git a/util/cpp/input.c b/util/cpp/input.c
index 135b45dab..6734b68bc 100644
--- a/util/cpp/input.c
+++ b/util/cpp/input.c
@@ -42,6 +42,9 @@ int	InputLevel;
 AtEoIT()
 {
 	/* if (NoUnstack) warning("unexpected EOF"); ??? */
+	/* This is wrong; in an #elif, NoUnstack can be set, but you
+	   can still get calls to AtEoIT().
+	*/
 	InputLevel--;
 	DoUnstack();
 	return 0;