Someone came into the restaurant I worked at during college FIVE MINUTES before closing time. They ordered a grilled cheese and chili. The grill had been shut down and the chili was already put back in the fridge. So, the guys in the kitchen stuck the chili in the micro, toasted a couple of pieces of bread in the toaster, then put cheese on the toast and put that in the micro. The people were there for over 45 minutes and had the nerve to look offended because we were vacuuming while they were trying to talk on their cell phone while they were taking their sweet time eating.

The worst thing, however, that any of the waitresses ever did to a crappy customer is switch people's drinks when they were refilling them. Say hello to your dining partner's saliva.

The worst I personally ever did to crappy customers was just take an extra 5 minutes to bring them their food/refills/ticket.