Shamelessly stolen from another site.
Mistaken Identity!
A bloke is in a queue at the supermarket when he notices that the rather dishy blonde behind him has just raised her hand and smiled hello to him.
He is rather taken aback that such a gorgeous woman 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 maybe mistaken, but I thought you might be the father of one of my children".
His mind shoots back to the one and only time he had been unfaithful.
"Christ" he says. "Are you that stripogram on my stag night that I shagged on the snooker table in front of all my mates whilst your mate whipped me with some wet celery and stuck a cucumber up my backside?
"No!!" she replies, "I'm your son's English teacher."
Mistaken Identity!
Mistaken Identity!
The Truth IS Out There, The lies are in your head. (T. Pratchett - 'Hogfather'))