Continuity mistake: When Grace is telling Will about his dad's affair, she mentions that he grabbed the other woman's ass when she brought him a packet of Goobers during the interval. The thing is, she sees them for the first time during the interval, and we see the conversation that follows. The mistress brings him wine, not Goobers, and he doesn't grab her ass.
Continuity mistake: When the guy at Will's party tells everyone the beer has gone, everybody leaves, and the guy puts his plastic cup on the handle sticking out from the keg. In the next shot the cup has suddenly gone, then reappears again in the next.
Continuity mistake: When Will tells Grace that he is gay, there is a tray of finger foods on the table next to him, as he kneels on the floor. In the second shot, the plate is covered in cling film, but in shots before and after, there was no cling film.