1. When people order drinks/food and then leave without eating or drinking any of it.
2. When people are 'drinking' from obviously empty cups. At least hold it like it's full of liquid instead of gesticulating wildly with it.
3. When characters or important back-story is introduced to serve one episode, and then is forgotten about forever after.
That empty cup thing annoys the hell out of me. It's like the actors don't even bother anymore. It's just so obvious. At least make the bottom weighted or something, or heaven forbid...actually put a drink in there!