The thing about carry out is that sometimes the order isn't what you asked for (orders get mixed up) or the food quality just is not good. Unlike eating inside, you don't find this out until later when you actually open up the bag.
Case in point: A companion and I went into a very high end respaurent and decided to get our food to go. He left a very generous tip. When we got home (the other side of town) we opened our bag and discovered they hadn't put any meat on either of the sandwiches. We were not pleased.
That's why I always check the food before I leave the restaurant, or at least the parking lot. That way if there's a mix-up, I can get it corrected while I'm still there.
(The only reason I do this is because that same thing has happened to me a couple of times, except I didn't know it until I got home, so now I'd rather check while I'm still there and get it corrected, if need be, before I leave the premises.)
Yeah, knowingly paying less than you owed because you didn't think the quality was good and you didn't care to talk to management is far worse than an etiquette breach.
POD. That means that either A) another table got and paid your bill, not noticing the price difference, which was more than what they owed and therefore unfair to them; B) the server had to pay the leftover portion of your bill, which was unfair to him/her, or C) the restaurant ended up just writing it off, which was unfair to the restaurant. Even with bad service, it's never OK to skip out on your check, either in part or in whole.