DUH X
From Reader's Digest   

A friend of mine works at a tattoo shop. A client walked in and got a sentence tattooed on his back. A few hours later, the customer called, demanding a refund.

Client: You did my tattoo backward!
Tattoo artist: It's backward?
Client: Yes! I'm looking at it in the mirror right now!
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After toiling over a 1,000-piece jigsaw puzzle, I was unable to finish it because there was one piece missing. I returned the box to the store and explained why.
As the clerk refunded my money, she asked, "How far did you get before you noticed there was a piece missing?"