On his way into the supermarket, a man walked by a cashier, who noticed that his fly was undone. Looking at him, she said, "Your barracks door is open."
Because this is not a phrase men normally use, he carried on walking into the store, feeling a little puzzled. Later, while shopping, a man came up and told him, "Your fly is open."
He zipped up and finished his shopping. Then, remembering what the cashier had told him, he finally understood. So he intentionally got into the line of the same cashier's checkout, the lady who told him about his "barracks door."
He was planning to have a little fun with her. So, when he reached her counter he said, "When you saw my barracks door open, did you see a soldier standing there at attention?"
The lady thought for a moment and said, "No. No I didn't. All I saw was a disabled veteran sitting on two duffel bags."