My favorite is when a self entitled crusty just assumes you're going to give them a special deal on something because "so-and-so was working the counter last time and did it for me!"
My favorite response is to tell them in a very even, monotone, voice that I am not "so and so." I inform them that were I to do something like that and store management found out I would most likely be fired.
While in truth I would probably only be verbally warned if I was caught doing something like that, it typically shuts the customer up pretty quick.
Rarely, it doesn't, and they give me the "I don't care if you get fired!" line. I just smile sweetly and inform them, "Well I do care, and I'm not going to get fired just to make you happy."