The Final Frontier (1) - S7-E21
Continuity mistake: In 2006, after Paul learns that Jamie is pregnant with their second child, Paul buys her a double baby carriage as a present. He says "Two kids, one carriage. One big happy family." A sweet gesture, but in the year 2006 Mabel would be 8 or 9 years old, far too old to ride in a stroller.
Plot hole: In this episode, a very star-struck Paul meets Jerry Seinfeld on the street. It's not clear whether Jerry is playing himself or his Seinfeld character, but either way it's a mistake. If Jerry was playing himself, it's a mistake because Mad About You did a crossover episode with Seinfeld in its first season, so it would be impossible for Seinfeld (the TV show) or Jerry Seinfeld the actor to exist in the Mad About You universe. If Jerry was playing his character, it's a mistake because it was established in that crossover that Paul and Jerry used to be neighbors and knew each other fairly well, so Paul wouldn't be at all star-struck.
Answer: Just think about your own life, I am sure you do not act the same with your siblings or close friends as you do with a stranger (because that is what the Buchmans essentially are to Ursula). In Mad About You, she is an extremely bad waitress and in Friends she is a jerk - those are not mutually exclusive character traits. This can also be seen in the Friends episode when Jamie and Fran are in Central Perk and run into Phoebe who answers their questions in an aloof manner and they assume it's Ursula. Is Phoebe always aloof? No. But she was to strangers asking her weird questions. Also, Ursula has been aloof with Phoebe as well (like in the one where they all turn 30 or the one where the grandmother passes away, etc).