If I tell you that you have to do X to get the card-reader to work, then do it, don't act like you know those bastard machines better than I do. I work with them all day every day.

If you're in an express lane, it's okay to ignore the limit if they're all the same barcode or if it's only by a few items. It's not okay to come in with a cart full because you're "in a hurry". I don't care if you're in a hurry, I didn't make you come to the store and buy a shit-ton of junk food, did I? You should have done it when you had the time to.

Customers who make comments to me about the customer ahead of them like "I bet that was a welfare-case." or "She was totally a tranny." (both of the things have been said to me). I just never know how to respond to things like that, so I normally just kind of shrug.