When "Buzzy Wuzzy" takes her bee mask with her and they leave. But when Will and Grace dive for the jawbreaker, the bee mask is back on the coffee table.
Position of silly string over the room.
When Will and Grace are taking the kids around the flats trick or treating, as Will and Grace enter the lift, the kids stand in the hall and their pumpkin bags have faces, but in the next shot they are plain orange. Also, When the father comes to pick up his children, the plastic pumpkin goody-bag moves from the edge to the center of the rug between shots.
When Jack and Karen are downtown on the streets, Karen wears a red shawl, and in one shot she pulls it up so it is over her shoulders, but in the next shot it is down by her elbows.
Position of the silly string bottle lid.
When Grace pushes the elevator button with her foot, the wall shakes, showing the wall to be a set.
As Will & Grace take Harlan's kids out of their apartment to go trick-or-treating, you can see on the left edge of the screen where the wall ends to the set on the sound stage where it's filmed.
Will refers to an incident when he and Grace were visiting her grandmother in Boca Raton (Florida). He mispronounces the city as Ra-tahn instead of Ra-tone.