A guy decides to take off work early and go to a bar. He stays their until it closes at 2 in the morning, at which time he is extremely drunk. When he gets back to his house, he doesn't want to wake anyone up, so he takes off his shoes and starts tiptoeing up the stairs. Halfway up the stairs, he loses his balance, falls over backwards, and lands flat on his rear end.
That wouldn't have been so bad, except that he had a couple of empty pint bottles in his back pockets and they broke. The broken glass cut his rear end badly, but he was so drunk he didn't know he was hurt.
A few minutes later, as he was undressing, he saw some blood. He checked himself out in the mirror and sure enough, his rear end was cut up something terrible. He repaired the damage as best he could under the circumstances and went to bed.
When he woke up, his head was throbbing and his rear was hurting. He was laying there trying to think up a good story, when his wife came into the bedroom.
"Well, you really tied one on last night," she said. "Where'd you go?"
"I worked late," he said, "and I stopped off for a couple of beers."
"A couple of beers? That's a laugh," she replied. "You were plastered last night - where'd you go?"
"What makes you so sure I got drunk last night?"
"Well," she replied, "my first clue was when I got up this morning and found a bunch of band-aids stuck to the mirror."