Most of the time if it was a small mistake (like if I ask for no onions and it comes back with onions) I will just fix it myself (e.g. take the onions off) and eat it.
There was only one time in my entire life that I've had to send something back.....
My then-bf and I were driving around with a specific place in mind, and passed a seafood restaurant on our way. He said he was in the mood for seafood, so we went ahead and thought we'd give that place a try. I was sort of new to the area and hadn't heard anything about that place yet.
It was a Saturday night at 8 pm and we didn't have a wait.. and there were plenty of empty tables which should have been our first clue, but we ordered an appetizer, some drinks and entrees anyway.
When our dinner came, I was appalled. The crab tasted old, and the fish did not even look like they even bothered to properly prepare it. I couldn't eat any of it. The chef came out and assured us that was the way the entrees were supposed to be prepared and I asked them to please take the entree away and asked for some more tortilla chips.
They only charged us for the appetizer and our drinks (the then-bf didn't eat his entree either) which was nice, but I will never ever ever go to that place again... and I tell everyone to never eat there.... and just in case you have one in your area, I wouldn't reccommend Rockfish to even my worst enemies