It's never fun to be a squeaky wheel, but whether you've been served an overcooked, undercooked, or just plain wrong dish, there's a proper and improper way to let your waiter know that you're less than satisfied with what's on your plate.
Foodie web site Slashfood lists various scenarios where the problem lies with the restaurant, including times when your meal is the wrong temperature, stale, has a foreign object (think hair) in it, or is not what you ordered. In all these cases, the fault lies squarely with the restaurant, which means that you are entirely within your rights to send the order back (that is to say, you shouldn't receive any complaints or trouble from your waiter).
If you decide to request a replacement, make sure to be specific about what you want changed. This means that before you ask for the waiter or manager, you should already know whether you want the meal to be fixed or cooked entirely from scratch. Or perhaps you want to swap for a new dish? Whatever your desire, first know, then relay it explicitly.
Check out the above video for more specifics on sending back food, then chime in with your own restaurant food-gone-wrong experiences and tips in the comments.
The Etiquette of Sending Back Meals [Slashfood]