This is crass, but accurate.
Any self respecting chef wouldn't dream of tampering with a customer's food.
In nearly 20 years I've only seen something untoward twice, both when I was a kid and was reluctant to speak out.
I'm not saying chef's don't make mistakes, far from it. The point I was trying to make was that a waiter could make a mistake - forgetting to put an order through, putting the wrong order through, put through an unreasonable amount of orders simultaneously, that would disrupt the kitchen's...
When I eat out I automatically have it in my head to tip, it's possibly some kind of professional empathy. If I don't then it's because something has gone considerably wrong, not just a petty gripe.
It's worth bearing in mind whether kitchen staff get a share of the tips as well :)