From my personal experience, any job you work that deals with customers, you're bound to run into the occasional one that will be ticked off for petty reasons.

For instance, I was doing work in the garden department where I work, and there was a customer who was demanding a VERY SPECIFIC request from the store. One that goes against the rules of the store. Nevertheless, they complied with the man's request, mainly because they knew the guy was bats**t insane. But he was trying to pay with a Macy's credit card, which got declined. About an hour later, he comes back with the items he asked to pay for, while not only cutting in line, but also, interrupting a transaction. He demanded the cashier to cancel the transaction she's working on just to pay for his, ALL WHILE THERE'S A BIG LINE OF PEOPLE WAITING TO PURCHASE. At that point, the man not only got ticked off at the cashier, but he started harassing her.