The Big Vent - S1-E9
Continuity mistake: When Grace is listening through the vent, she is leaning on a cushion. Between shots the cushion rotates because the stripes on it change from running horizontical to vertical. (00:43:45)
The Big Vent - S1-E9
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)