The summer camp where I used to work would have bunks rotate to be on dining hall duty. It basically just meant serving food as people came through the line, and cleaning up afterwards. Every Sunday, I would do something kind of mean but do not regret it at all.
Every Sunday at lunch, the oldest campers would make deli sandwiches for everyone. The first step was asking for either a roll, sliced bread, or a wrap. I would go up and ask for a roll and when they gave me one I would very angrily say, "That's a bun!" According to the FDA, there is no difference between a roll and a bun, but the kids didn't know that. So they would frantically look at their assortment before I said, "Ugh. Forget it!" and stormed off.
I did this every week for years and no one ever caught on. I think I can safely say I am someone's worst customer, and I am okay with that. That said, I have nothing on these horrible customers, as relayed by their waiters and waitresses of Reddit: