Continuity mistake: In the 1991 flashback, Jamie is already engaged to Paul and is shown with long hair. However, in the 'Cold Feet' episode, Jamie is shown with short hair. Since the 'Cold Feet' episode was set in February of '91 and the flashback from this episode was set in late March or April of '91, these two episodes don't match up.
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.
Continuity mistake: Jamie and Paul are at the supermarket, and Jamie is eating chips. She leans on a coin operated horse. The bag of chips in her right hand changes from front (Green Label, "Baked Lays") to back view in one shot then to front view again.
Deliberate mistake: The first 10 episodes of Season 5 were shot in a totally different order than they were aired. As a result, Jamie's size during her pregnancy is very inconsistent at the beginning of the season. In particular, she gets much smaller between episode 8 and episode 9 because episode 9 was originally supposed to air earlier in the season.
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).