Back when I was in college, I worked in a supermarket, as a cashier, for two years. The hours were long, and the pay was crap. Standing on my feet for 10 hours a day didn't help either. To this day, I maintain that it was one of the hardest jobs I've ever had. And it wasn't because the job functions were particularly complicated, they weren't. It was a tough job because I was dealing with a constant roulette of difficult, self-entitled people.
On a daily basis, I had to deal with some incredibly frustrating scenarios. If you've ever had the displeasure of being a cashier, you will be able to relate to all of these.