too many to list...
... but here are some good ones.

Phone conversation:
Me: Hello?
Stranger: Oh, I must have the wrong number.
Me: OK, bye!
Stranger: I was looking for ADT Security.
Me: Yes, you do have the wrong number, bye!
Stranger: Do you have their phone number?

A favorite from working in a rather large bookshop:
Where is the nonfiction section?
(I once replied "it's the bit of the store outside the fiction section.")

Overheard in a convenience store:
Middle-aged woman to very old man: It was the greatest film ever! You have to rent it. The Wayans Brothers actually DRESS UP as WHITE CHICKS!
(emgee, Fri 7 Jan 2005, 20:10, Reply)