Continuity mistake: In season 3, Episode 9, "Holy Matrimony": When Kitty and Robert get married, you see Kitty putting the ring on his finger whilst speaking the vows. Then in the next segment once they have been pronounced man and wife, Robert goes to kiss Kitty and puts his left hand up to her face he has no wedding ring on.
Other mistake: In Season 2, Episode 14, "Double Negative" during the scene at the fundraiser, Kitty arrives and asks what the quickest way to a glass of wine is, to which Sarah (who is standing opposite her) offers her her own drink. Calista Flockhart (Kitty) does not see this, and another cast member attempts to take it from her, but Flockhart finally notices it and smirks at the realisation that she messed up.