You have the right not to have your food messed with
Let’s get one thing clear: I don’t care how dickish a customer is. I don’t care if you know they’re going to tip badly. I don’t care if they aggressively fart at you and tell you your mother has carnal relations with dead goats. You do not get to spit in or mess with their food.
That being said... this doesn’t really happen. Oh, I’m sure it has happened before, on occasion, somewhere, but most people who’ve never worked in a restaurant assume it happens all the time (I’ve had restaurant story submissions where customers assume it’s going to happen for everything up to and including looking at the server with a mildly perplexed expression), and that’s just not true.
Blame whatever you like for this borked perception (personally, I blame the movie Waiting), but in about six years of waiting tables, I saw or heard about a server spit in a customer’s food exactly zero times. And honestly, every server I’ve ever known who is also a decent human being is horrified just by the entire concept. I’ve talked to a lot of fellow servers about this, and I know one who had seen it happen -- and in that case, the food never made it to the customer and the server got fired for it. Even still, it’s important to note that this is Not OK.