I hate being able to tell that the coffee cups on screen in a TV show or movie are empty. Actors come in carrying two in one hand at odd angles, set them down quickly and heavily, someone else picks one up and tilts it almost horizontal to sip from it when it's supposed to be fresh and new. It's just a detail I notice that pulls me out of the action and says "this thing you're watching is fake".
The thing is, the fix is super-easy: fill them with water. For any other beverage container - a wine glass, a regular drinking glass, a bottle of water - directors or props people put something in them. For some reason coffee cups are immune to this treatment and are carried empty. No doubt it saves them several seconds of preparation and avoids spills, but I think the main motivation is "it won't be visible, so don't bother". And they're probably stretched pretty thin already to bother with details like this. Still, if I were in charge, I'd make sure the coffee cups were filled, because it's a pet peeve of mine.
Mokalus of Borg
PS - Perhaps some props people do fill the cups.
PPS - And perhaps I never noticed those because it looked normal.