Continuity: In episode 1:12, "My Fair Maidy", when Will introduces Grace to April, the pens in Grace's hair changes between shots.
Continuity: In the Pilot, when Jack and Will are playing cards with two other guys, Will deals Jack his first card two times between shots.
Continuity: In "Boo Humbug", Harlin's daughter takes her bumblebee mask with her when she leaves. A few shots later, when Grace and Will dive for the candy, it is on the coffee table again.
Factual errors: The layout of Will's apartment, specifically his bedroom and the bathroom(s), don't match up with the layout of the hallway outside his door.
Revealing mistakes: In the earlier series, the deadlock on Grace's office door is backwards. The door opens inwards yet the part of the lock where the key is required is on the inside of the office and the latch that can be opened by hand is on the outside - rendering the lock useless. This is corrected in later seasons.