Continuity mistake: When Jack and Sissy are sitting in his apartment, discussing what they've done with their lives, they both have mint cookies that they eat while they talk. The bites and size of their cookies, most noticeably Sissy's, change between shots.
Continuity mistake: At the club, Will is at the bar and he says "another round," and in the next shot a beer and a yellow Alco-pop drink is on the bar in front of him. In the next shot they've vanished, and the bartender once again places the same drinks down again for him. (01:16:40)
Continuity mistake: After letting Campbell out, Grace pulls the white blind down the door, and it stops about halfway down the door, and gets stuck, and is still rolled at the bottom. In the very next shot, the white blind is now unrolled all the way down perfectly. (00:07:00)
Continuity mistake: When Will enters Barry's apartment with the big brown bag of Chinese food, he walks over to the chair and puts it down. On the table is a magazine laying parallel to a book. A shot later the magazine sits vertically to the book.
Continuity mistake: In the episode "Grace, Replaced", with the neighbor Val who moves into "apartment 15F" in the same building as Will and Grace. Yet in the previous episode "Yours, Mine or Ours" with Peter (who moved into 12D) we learn that 12 is the highest floor in the building...