At an Apple retail store this weekend (Tysons Corner) I mumbled to myself aloud, "I wonder why this scanner is so big" (it was a few inches thick). A store employee chimed in with a, "What?" I said, "I have a Canoscan that's an inch thick. It's too bad they can't all be so thin." His reply: "It's bigger because this one uses FireWire."
Usually it's the customer that says something stupid...
And on a related note, check out
http://www.rinkworks.com/stupid for some pretty funny things people have said when not understanding computers. Maybe I can add this one.
-Rob