I think its important we make the distinction here that I'm not talking about when food is over/under-cooked. If something your ordered does not come as you ordered it, I think the rule is you send it back. Despite all the horror stories we've heard about what cooks do to returned food, I haven't ever met anyone (cook, chef, even fry cook) who actually messes with the food (as much as they probably want to). Flashback to Roadtrip and the French Toast scene comes to mind here.
However, I still think that in most restaurants--and by most, I mean anywhere that your dish costs more than $8.99--it is alright to tell the waiter/waitress that your meal does not taste as expected. AND, they should allow you to order something else. Am I the only one who thinks this way?