I'm a waiter and ive done the same thing. you can add messages for the cooks, usually an addition/ subtraction to the food, and every once in a while they get printed out on the ticket. I dunno why it happens those few times. I was lucky enough to catch mine before it went out to the table but it could have been different. the bottom line is, waiters deal with alot of bullshit and alot of dumb people so you develop a dark sense of humor. I can say that ive said a million rude, crude things about people but ive never done anything to their food/ drinks. If i didnt get it off my chest by joking id definitely be tempted to get revenge in other ways.