When I tip I tip based on the service that the server actually provides and their general attitude. So, if the meal is late or burned or whatever, but it is not the fault of the server, then I don't penalize the server for that. In fact, depending on how they handled it, I might leave a larger tip. I would tip anywhere from zero on up to astronomical percentages based on many different things. I'm not afraid to tip zero.
Just out of curiosity though, how do you feel about leaving tips when doing pick ups where you go in yourself? I will leave a tip if they come out to my car but I leave zero tip if I have to go in and pick up the food. I figure I don't tip a McDonalds counter person so why should I tip a counter person at a real restaurant?