In the scene where Monica is at the dance with "college boy". She says that her mouth is dry, so "college boy" gets her a drink, but when he comes back he has no drink and he asks her to dance...where was the drink??
I've noticed that one too, its pretty obvious. I don't know that I would call it necessarily a slip-up though. It could be that when he heard the slow song come on he decided not to get the drink and go ask her to dance because at that point in time Q is talking with her. "College Boy" might be trying to get Monica away from Q so he could have her attention.