Used to work in Debenhams...
One woman bought half the sale bin. 90% didn't have a barcode so needed to type the SKU in. She returned it all the next day.
"Can I return this? No, it's not been worn...."
"Can I return this thing I bought yesterday?" No receipt, no tags, four year old stock, I know we didn't have any in the sale that year and they want the full original price refunded.
Not the customers fault - but customer not happy with refund policy, demands to speak to manager, manager on the phone tells you to give them what they want.
Customers who throw stuff at you/shout at you/generally behave inappropriately.
Gold card holders thinking they could push to the front of the queue. Nope, just meant you spent £500 in six months or something.
People who leave their small children unattended on the toy department then get shirty when they're called out on it. Sometimes they'd actually left the store. Or the three year old who'd had their five year old brother left in charge of her was memorable. Brother wandered off. I found the three year old, who didn't know her surname or her parents names so I ended up carrying her about the whole floor, talking about Tweenies and asking her to point if she saw daddy. For half an hour. Then her parents turned up and claimed they'd only been on the next department🙄