Food contamination and potentially spreading diseases are not worth the bittersweet revenge of watching a rude customer gag, but the rage caused by someone belittling your entire sense of self and human existence over their missing free-refill of soda is definitely strong enough to fuel some dreams about it. Tiny, safe tampering such as not giving the demon at table 4 the freshest, prettiest vegetables for their salad or the extra scoop of ice cream in the birthday sundae you usually give your regulars can be just as satisfying sometimes. It's always in your best interest as a customer to be as nice as possible to the people controlling your food and alcohol for the evening, because the same people that wish they could spit in your extra well done steak could also give you those free top shelf tequila shots.
So, when a Reddit user asked restaurant workers, "what is the worst thing you have seen done to a customer’s order?" people with secret stories from the kitchen were ready to spill the nasty details.