The worst one I ever witnessed was at this sushi bar, and this customer sat nearby at the bar. He tells the sushi chef that he doesn't like a list of ingredients in his roll, so please substitute for something else.
This chef was crazy busy, and, in my opinion, what he was asking for was impossible!
I don't remember what he didn't like, but one of the things he ordered was literally everything he didn't like! For example, asking for a spicy tuna roll and not liking tuna. The chef said he would have to substitute everything, and the customer said okay. "So how am I going to make this roll?"
The chef said he would have to substitute everything, and the customer said okay. "So how am I going to make this roll?" He was frustrated! He was probably helping six people in his section and multiple tables at the same time! I could say 7, because I probably ordered enough for 2 people...