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...
Continuity mistake: When Jack is trying to hush the audience at his show Jack-Talk after bringing Grace onto the show for a follow-up interview, between shots the little bell on his desk moves from his right (near the edge) to over past his left, near the centre of the desk with all the other items. (00:15:55)
Continuity mistake: When Grace is setting up her design class, she writes "Professor" on the board, and curls the end of the R into a spiral. But in the shot when she says "T.A. - Teachers Assistant" the spiralled end on the R has gone. In the next shot it's back again. (01:04:40)
Continuity mistake: Sharon Timmer introduces herself to Grace in Series 10 Episode 1: "I've lived directly below you for 19 years." But then in Series 11 Episode 3 she seems to live in Flat 9B next door to Will and Grace. I'm not sure we've ever seen who lives there before, but guessing by the layout of Will and Grace's flat, there wouldn't actually have been space for an extra flat in that space. (00:00:16 - 00:12:12)