Yuck to the PP who had a customer try to hand them money from their underwear! I thought it was bad enough when I had a customer (who was very obviously a drug user and high on something at the time) hand me slightly damp money from their sock (and, yes, I took it as I worried about what he'd do if I didn't)!
The retail chain I used to work in often had coupons in the paper on a Saturday that gave discounts on a certain product (the product would vary week to week). It was always really popular and generally by the time it got to midday/early afternoon we'd be sold out of the product or at least only have limited colours/scents etc. left.
But you'd always get customer INSISTING we 'check the back room' for more of the product/specific shade. I'd explain, very clearly and politely, that we had DEFINITELY run out as multiple customers before them had tried to buy the same product so we'd checked very thoroughly and could see on our system that we were out. They'd still insist.
If you ever do this, please know the shop assistant NEVER actually checks. They will always walk into the back room and stand around for 30 seconds or so before walking back out and declaring they couldn't find more. The smug 'well, at least I made you check' look really does just make you look like a stupid twat!
I once had a customer scream at me when I asked if she needed a bag (for one very small item) because clearly I was trying to make her look like a shoplifter and if she didn't take a bag I would then stop her at the door and call the police. I I had given her a receipt...
It was always amusing when customers would threaten to take their custom elsewhere! Oh, no! I'm sure the big national chain of stores I work for will really notice you not buying a bottle of shampoo and tube of toothpaste from them once every couple of months anymore!
If they threatened such a thing (especially if the reason they were annoyed was because I wouldn't put up with them swearing at cashiers - I was a supervisor) I would sweetly point them towards the very similar - but more expensive - chain that had a store just up the street from us. That made them furious.
If a customer was nasty/rude I was an absolute stickler for the rules. If they came in demanding a refund on something without the receipt then I'd shrug and let them know my hands were tied as it was against our policy.
If, however, they were pleasant and polite then I'd go out of my way to help/bend rules etc. because I thought that behaviour should be rewarded! I had a woman ask for a specific shade of make-up once. We didn't have any but did have a couple of testers. I explained that the testers didn't have a barcode and we weren't supposed to sell them.
She was so nice and funny and told me that clearly I didn't need the make-up being the fresh-faced beautiful young lady I was, but that she really needed to slap in on with a trowel and she'd be soooo grateful if there was any chance I could find a way to bend the rules. I was so used to customers just screaming at me that I put it through for her and just asked her not to tell anyone! A bit of humour and flattery can get you a long way with people who are used to dealing with cunts!