Those Barrack Doors...
A man walked into a supermarket with his trouser zip 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 almost finished 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 queue 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 soldier in there standing to 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 old sod sitting on two old duffel bags headed home for retirement."