Revealing mistake: As Will and Jack walk along the street, out of NBC, they stop and Will says "Once you have a weatherman on your side ." you can see people across the street staring at the crew and actors, and one man even takes a photo.
Visible crew/equipment: As Will and Jack walk along the street, out of NBC, in one shot as they walk along you can see the cameraman with a camera over his shoulder walking along with them, reflected in the store windows.
Continuity mistake: When Will and Grace walk down the street in the opening credits, in every shot change the people walking around and behind them on the streets change, and the cars on the street change too.
Continuity mistake: When Grace is talking to Karen about how she has just come out of a string of bad relationships, she picks up a book of cloth samples, then a second one. In the next shot her hand is empty and she picks them both up again.