Do waiters and waitresses really spit (and other things) in your food if you treat them poorly?

I’ve heard tons of waiters and waitresses caution against being too mean to them, lest one end up with a sneeze muffin (at best). But here’s where it all falls apart for me: When I was working as a cook, if anyone had ever found out that I did that to someone’s food, I would have been fired immediately. Hell, I would have been fired for accidentally forgetting to take off a ring, thus bringing in dirty under the ring to the food. And the restaurant would have gotten some seriously huge fines, if not gotten shut down. So is this just a threat than has continued from days when you really could do that, or have the laws and enforcement made it so that this is an empty threat designed to keep customers in line?