I've never worked in retail but I do know a lot of people who have. Of course there's the usual crabby people and people who think talking on their cellphone while they're at the counter is acceptable and poop in places it should never be (toilet seat, sink, ceiling, etc.), but my favorite story is when my friend was working at a grocery store.
One time she was working nights at this grocery store and this dude decides to pull his junk out and walk around the store for a while. Everyone just tried to ignore him and hoped he would leave, but apparently that wasn't enough for him so he went to check out. He put it right on the checkout counter, and when he went to pay he tried to get my friend to take the money from his hand near there. She refused to look down and made him hand her the money instead. She handled that better than a lot of people would.