So let’s say that you’re sitting at a restaurant and when your food arrives, you take one bite and decide that something is preventing you from taking a second. No matter what, this dish is going back to the kitchen, and now it’s up to you to find a way to send it back politely.
First of all, the single most important thing to do when you’re sending something back is to do it immediately. If you eat half the dish before sending it back, the server and kitchen will (rightfully) believe that you really didn’t need to send it back.
If you asked for your dish to be prepared with no tomatoes and it’s served with tomatoes, you can either decide to pick them out or you can send it back and remind the server politely of your special request. No big deal, they’ll take care of it.
If a dish is unbearably salty or a salad is so overdressed that it’s inedible, just let the server know what the situation is, and they’ll take it back to the kitchen and bring you a new one. Remember that it’s not the server’s fault, so don’t take it out on them. Also, don’t be afraid that they’ll get angry at you, or someone will spit in your food. It’s their job to make sure that you’re satisfied (within reason), and if the kitchen clearly made a mistake the server will be frustrated with the kitchen, not with the diner who’s been served an inedible meal. A new dish will be fired, and nobody’s going to spit in it.
There is, however, one instance when sending back a dish will really annoy both the server and the kitchen. If you order a pasta dish that contains pancetta but then send it back to the kitchen because you’re a vegetarian and didn’t know what pancetta was, then congratulations because you’re one of the most unbearable types of customers a restaurant can have. When you send a dish back because of incompetence in the kitchen it’s understandable; When you send one back because of your own incompetence it isn’t. A perfectly good plate of food was just thrown out because you didn’t ask what pancetta is before ordering.
So the bottom line: Use common sense if you think that you need to send your dish back. If the kitchen did something wrong, then they’re on the hook for a new plate of food. If you did something wrong, then hopefully you’ve learned a lesson.