Continuity mistake: When Grace finally decides to move into Will's apartment near the end, we see Will eating a tub of ice cream. Between shots the ice cream tub lid jumps from being on the table behind Will, to suddenly sitting right in front of him.
Continuity mistake: When eating in Will's office, Grace eats salad from a plastic container. Through the scene the salad in the container changes colour, position etc. Also between shots, lettuce appears on Grace's fork.
Visible crew/equipment: When Grace asks Will for his advice on where her mirror in her new apartment should go, she goes running to the wall and when she does, the camera pans over and you can see that the wall stops, revealing it to be a set.