Here's a true story:
The wife of one of my colleagues at work has a part-time job as a checkout clerk at a local Safeway store. Well, a customer was questioning the price of a box of Tampax she'd put in the cart. So, Bill's wife calls over one of the young lads doing the bagging to go check the price.
By this time, there's a lineup at the checkout and the young fella returns and, in a loud voice, says, "Did you want the ones you push in with your thumb, or the ones you smack in with a hammer", innocently thinking that Bill's wife had asked for a price check on a pack of thumbtacks.
You can imagine the embarrassment on the face of the lady customer and the barely concealed guffaws from the rest of the lineup.