close the barn door after the horse gets out—not take any action until after a problem has already occurred:
- The problem is that post-accident testing closes the barn door after the horse is long gone.
your barn door is open—(U.S. euph.) is used as a caution to a man having his trouser zip undone:
- “Excuse me … I couldn’t help but notice your barn door is open.” “What?” “Um … your fly is open.”