This has been seen in lots of livejournals, so if you posted this in yours already, I may have seen it there:
A guy goes to the supermarket and notices an attractive woman waving at him. She says hello. He’s rather taken aback because he can’t place where he knows her from. So he says, “Do you know me?” To which she replies, “I think you’re the father of one of my kids.” Now his mind travels back to the only time he has ever been unfaithful to his wife and says, “My God, are you the stripper from my bachelor party that I made love to on the pool table with all my buddies watching, while your partner whipped my butt with wet celery???”
She looks into his eyes and says calmly, “No, I’m your son’s teacher.”