Sometimes memory lapses are a good thing
A guy is in line at the supermarket when he notices that a rather dishy blonde behind him has just raised her hand and smiled hello to him. He is rather taken aback that such a looker would be waving to him, and although familiar he can't place where he might know her from, so he says "Sorry, do you know me?"
She replies "I think you're the father of one of my children."
His mind shoots back to the one and only time he has been unfaithful. "Holy shit," he says, "are you that stripper from my bachelor party that I screwed on the pool table in front of all my friends while your girlfriend whipped me with some wet celery and stuck a cucumber up my butt?"
"No," she replies, "I'm your son's English Teacher.