Plot hole: Ellen and Rob wear wedding rings, yet their wedding is not announced until "An Affair to Forget" in the second season.
Continuity mistake: In the poker scene at the beginning of the pilot we witness Will, Jack, Rob, and others playing cards. Shortly after Jack asks for "four cards please" watch the bottom left of the screen as a hand (presumably Will's) deals Jack a single card. In the next shot Will appears to have not dealt Jack any cards yet and when he does he gives Jack five cards, add that to the single card he gave in the previous shot and the Ace Jack kept from his original hand and we get a total of seven cards. Two more than Jack should have had if the hand was dealt correctly.
Continuity mistake: Following the argument between Will and Grace when she returns from City Hall, Karen tells Grace she "thinks it's time for a couple of blue bibbies." Watch closely as she removes the lid from the pill bottle, takes out some pills, and puts the lid back on. The scene shoots to Grace and we hear Karen shaking the closed bottle. When the scene shoots back to Karen we see that the pill bottle is open again and Karen closes it yet again.
