I found this at 22 Words, but its origin is obscure:
Me: Can I use the bathroom?
Teacher: I don’t know, can you?
Me: When I said “can” I was using its secondary modal form as a verbal modifier asking for permission, as opposed to expressing an ability. I thought since you were a teacher you’d know that. My bad. May I use the restroom?
AB — 19 April 2011