I was in a dance studio that had signs up advertising their upcoming recital. The flyers said: Tickets on sale now. I overheard a girl (about 10 yeard old) ask the person in charge: If they're on sale now, how much did they used to cost? The person laughed and explained that in this case "on sale" just meant "for sale."

Is there some kind of linguistic/grammatical explanation for the misunderstanding or is it a legitimate misread by a kid who clearly goes shopping too much? :)