Continuity mistake: When Jack meets his father in the restaurant, his father says that he got a phone call from Karen Walker. When Jack says, "Wow," there is no sign of his arms being crossed, but when it cuts and Jack says, "I got the same phone call," his arms are suddenly crossed.
Visible crew/equipment: In most of Season One at least (choosing a specific moment here), Will and Grace have a large ornamental silver ball in their living room, which always reflects the studio lights and the fact that there is no fourth wall to the apartment.
xx:xx:xxHe's My Brother
Continuity mistake: When Will is cleaning the tray, and in all following shots, from when he leaves the room, to when Karen walks over to Grace, to tell her to go get Campbell, there is a wine bottle which keeps jumping from the edge of the blue sink counter, to the edge of the counter which has the cooking gas-hobs on it.