...or what kinds of things to they have to get wrong, for you to send it back?
I was having lunch in Little Saigon and ordered pho with rare beef and tendon (no mistake in ordering, I'm sure - the waiter even repeated it back to me in Vietnamese AND English). I got rare beef and tripe, which I don't really like at all. But I ate it anyway, thinking in the moment that the entire bowl was probably going to go in the trash. It's not about money because this resto's not hurting for business, but I guess I weighed the conservation to the minor dissatisfaction of eating tripe and opted to eat the tripe.
On the other hand, if I order a steak medium rare to rare and get something medium to well, that's almost certainly going back. Obviously, I find a well done steak a lot yuckier than tripe. :P
When do you just suck it up and eat it? When do you send it back?