A man walked into a supermarket with his zipper down and his fly wide open. A lady cashier walked up to him and said, "Your barracks door is open."
This isn't a phrase men normally use, so 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 and remembering what the cashier had told him, finally understood. He then intentionally got in the line to check out where the lady was that told him about his "barracks door".
He was planning to have a little fun with her. When he reached her counter he said, "When you saw my barracks door open did you see a soldier standing in there at attention?"
The lady thought for a moment and said, "No, I didn't. All I saw was a disabled veteran sitting on two duffel bags."