I worked as a cook while in high school and college. The people I worked with wouldn't even think of despoiling someone's meal when I was around. They knew better. What happened when I wasn't there is another matter.
One of the strangest customers I ever ran across was an older woman who returned a t-bone because it was overcooked. She had ordered the steak black and blue, where the outer portion is seared and the inner part very rare.
The steak was cooked perfectly. The manager told me to put another one on the grill, count to ten, flip it over, count to ten again, and he'd serve the meal himself.
I was certain the woman would complain that the steak was now undercooked; in fact, it was raw.
Some time later, the manager returned and said, "Come here, I want to show you something."
We walked into the dining room, where he pointed to a table. "I think she liked her steak," he said.
The woman had eaten every scrap of meat off her plate, even the fat, and was busy gnawing on the bone like a dog.