I used to work in the electronics department, but I got reassigned to checkouts about a year ago. Patience is not something I have a lot of, so it's often a struggle. I have customers who truly consider me to be a slave - after all, they're paying customers, and I have to do anything to keep their business.
One of the most bizarre requests a customer made of me: a customer was a few dollars short of the total owed, so she told me to pay the rest of it with my own money, and then she started to grab her bags and walk away. I held onto the bags and told her that I was not going to pay for customer's purchases with my own money and that she either had to pay the full amount or remove some merchandise from her purchase. After screaming until she was literally red in the face and calling me every name in the book, she abandoned the purchase altogether, yelling about how horrible my customer service was for not "helping" her and "giving her a break."
A more frequent problem is when customers don't know how they want their merchandise bagged. I'm willing to bag things the way people want me to (I'm particular about bagging when I'm a customer, too). However, when customers change their minds several times and expect me to unbag everything just to rebag it again, all the while holding up my line with a growing queue of impatient customers behind them... that's too much. One customer had me bag, unbag, and rebag no fewer than six times, after which I pointed out that I had a growing line of customers that needed attention, and that I was going to put the bags in the customer's carriage, along with some extras in case they were needed, so the customer could bag his items the way he wanted them. Well, that stopped everything cold, and it got so bad I had to tell my waiting customers they might actually get faster service in other lines. I had to call the manager, and the customer insisted that I refused to bag his merchandise outright. Fortunately, the manager knows I'm not a nut, so the security tape was reviewed, and the customer was then told that I had, in fact, bagged and rebagged six times already. The customer said he didn't care about that - it was my refusal on the SEVENTH time that made him angry.