The saying "the customer is always right" has long-dominated the food service industry, and for good reason: the people who say it are the owners of the restaurant, NOT the waiters.
Waiting tables is hard, mentally exhausting, backbreaking work. You have to manage a lot of detail, stay on your feet for long periods at a time, carry heavy and sometimes scalding hot serving plates. On top of it all, you have to do with with a SMILE on your face, lest the person you are waiting on decides to stiff you on your tip, an amount of money that your salary has been adjusted downward to account for.
Because of this dependence on tips, oftentimes waiters won't tell you if you're being an obnoxious P.O.S., and they will, in certain extreme circumstances, take matters into their own hands and do gross stuff to your food. We went into a popular Manhattan restaurant and talked to real servers, who while they would never admit to spitting on anybody's food, did dish on the behaviors of customers that would make them WANT to do that.
So take a look at this gallery and try to avoid said behaviors at all costs. 'Cause if you don't, you never know what you might be eating…