Continuity mistake: When Grace reads Dr. Morty's letter on Karen's boat (in the last episode of season 5) it sounds at least a few paragraphs in length. During this episode, when she holds the letter up there is only a short paragraph on the bordered side of the page, and nothing on the back side.
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. (00:46:40)
Continuity mistake: After giving Will and Grace his scripts to read, Jack places his drinks bottle on the table and walks over to the door. When he reaches the door he suddenly has the drinks bottle in his hand, although he left it on the table across the room. (00:47:00)
Continuity mistake: As Leo speaks with Diane (he's on the sofa; she's in a chair) she refills her wine glass until it is nearly full. In the next shot Diane takes the last sip of wine from her glass, and the following shot shows her glass nearly full again. (00:08:45)