There seems to be a particular type of person who works in the kitchen at certain restaurants. I'm not talking chefs, here - I'm talking minimum-wage burger flippers or greasy spoon cooks. In the town in which I grew up, there were two middle-of-the-road restaurants, and one pricier, more upscale place. I worked at one of the middle-of-the-road restaurants, and knew a lot of people who worked at the other joint. In fact, a lot of the cooks would quit (or more likely, get fired) from one place and go to work at the other. So I feel pretty confident in saying that the same types of "pranks" went on at both places.
One memorable incident happened when the waitress reported to the cook that "so-and-so" had come in. I didn't recognize the customer's name, but apparently the cook did and he looked extremely pissed off. He asked the waitress, "Is this his order?" holding up the customer's ticket - a BLT - and the waitress confirmed it. After she left the kitchen, the cook (I'll him Bob) took the bacon off the grill, threw it on the floor, and stomped on it. He commented while he was doing it that those were the same shoes that he wore out in the yard where his dog sh!t. He wouldn't listen to me when I told him to stop it, and just started laughing. So I intercepted the waitress and told her not to serve that sandwich to the customer, and why. It got back to the manager (I can't remember if I told him or if the waitress did - it doesn't really matter anymore) and Bob got in huge trouble. Didn't get fired, though why he didn't is beyond me. If I was the manager, he'd have been gone that second. And then Bob had the balls to ream me out for "telling on him".
Bob did eventually get fired, after many further offenses, and his replacement came in the form of "Arthur". Arthur was an old buddy of the restaurant's owner. And by old, I do mean old. He was easily 70, 75 years old, and wasn't the sharpest tool in the shed anymore. He was a nice guy, but extremely slow and was very aggravating to work with on a busy Friday or Saturday night. I was a fry cook on those busy nights, and my job was to time my stuff to be done along with his stuff. It was excruciating to see the tickets piling up while I was standing around unable to do anything, because Arthur was puttering around in back instead of tending to the grill. One night, Arthur left the hash browns sitting out all night and I caught hell from the owner the next morning because they were all rotten. Of course, nothing was ever Arthur's fault, because the owner and him went waaay back.
Friday nights were fish fry night. One night, I witnessed Arthur absentmindedly dump a whole fryer of freshly-fried fish into the garbage can. When he realized what he'd done, he simply picked them back out of the garbage and put them on the customer's plates. One of the other cooks there, when something like that would happen, he'd throw them in the fryer for a couple of seconds and state that the oil was so hot, it would kill any of the germs picked up from the garbage. Uh, yeah, maybe - but what about all the rotten food in the garbage that is now fried onto the fish? Ewww.
Oh, and just a tip - most cooks take it personally if you send food back and there's a good chance they'll do something disgusting to your food. I witnessed Bob, more than one time, spit on someone's food because he was angry that the customer had complained. (And yes, I intercepted that food and reported it as well. And no, he didn't get fired for that either.) I have never sent anything back to the kitchen, and don't ever plan on doing so. I'd rather just eat it (or not eat it) in silence and not come back to the restaurant.
Surprisingly, I still eat out in spite of knowing what goes on behind the scenes at many, if not most, restaurants. But I do have to work hard at not thinking about what went into making the food.