A man walked into a supermarket with his zipper down. A lady cashier walked up to him and said, "Your barracks door is open"
Not a phrase that men normally use, he went on his way looking a bit puzzled.
When he was about done shopping, a man came up and said, "Your fly is open."
He zipped up and finished his shopping.
At the checkout, he intentionally got in the line where the lady was that told him about his "barracks door." He was planning to have a little fun with her, so when he reached the counter he said, "When you saw my barracks door open, did you see a Marine standing in there at attention?"
The lady (naturally smarter than the man) thought for a moment and said "No, no I didn't. All I saw was a disabled veteran sitting on a couple of old duffel bags."