yeah, it really depends on how it happens.

if i receive some other dish that also pleases me, im not sending it back cause i dont want it to go to waste. and if its cheaper than what i had ordered, im paying the price of what i got, at most. If what I receive is shit food, i really dont mind sending it back and it going in the trash or whatever. I believe the restaurant that takes a plate that someone has already taken a bite of and reserves it to someone else at a discount is infringing some sanitary law… at least where i live.