Ahh... the good old food service days.
1) As a busboy, I saw all kinds of nasty shit. I worked at a Bill Knapps when I was 15. For those of you who don't know, it was a real popular joint among old people. Cranky old people. I saw food get spit on, thrown on the floor and served, goobers, pubes, you name it. It was disgusting. This was the cooks' only way of leveling the playing field in their shitty world, and all you had to do to be punished was show up at the wrong time.
2) As a short order cook in a huge grocery chain - which I really enjoyed doing as a kid - I never did any of that crap. However, given that I served $2 cheeseburgers, if you made me make the damn thing 3 times because I hadn't yet hit "medium rare" perfectly on target, you were going to get a raw slab of meat back. Nuked charcoal in the case of "medium well."
Almost forgot:
3) Worked briefly as a waiter in an upscale Italian restaurant. The head chef made six figures in this place. I never saw or heard of any tampering of any sort go on.
But yeah, never EVER send food back. If you absolutely cannot force it down, ask to have it taken away and removed from your bill. Do NOT accept a replacement.
My dad would send eggs back for being the wrong shade of white. God only knows what he consumed over the years.