Corrected entry: Not really a mistake, but notice that the auditioning wedding singer from the first film and the foreign man that buys the Banks' house in the second film are the same man.
Corrected entry: When Nina and George are discussing Annie's baby news, Nina figures out Annie's due date is July 29th. Later, when Nina finds out she's pregnant herself, she and George figure her due date to be Labor Day. How can both mother and daughter go into delivery at the same time when there is over a month in between each due date?
Correction: Nina gave birth prematurely.
Corrected entry: In the scene when Annie and Brian visit George and Nina's house to share some news with the both sets of parents, they blow the horn in their car. Less than 1 second after the horn honks, they are seen walking through the front door. Impossible for them to have exited the car and crossed that distance in time shown on screen.
Correction: True, the horn does honk just before they walk in, but many vehicles come standard with "keyless entry" and also come equipped with a "door lock" button on the same device. The car's horn (as well as the headlights/tail lights flashing) is almost always connected to the alarm feature that is associated with remotely locking the car. It appears that they were just locking the car before entering George and Nina's house.
Corrected entry: When Annie and Brian get into a fight and Annie comes over to her dad's house, she tells her dad she got into her car and left. Then, after they talk for a while you hear Brian pull up in his car. When they are leaving you only see one car. So what happened to the other one?
Correction: They could've easily left it at her parents house.
Corrected entry: In the first movie, Brian's parents give them a sporty convertible for a wedding present, but in the second movie, they drive a jeep. What happend to the first car?
Correction: The second movie is quite a few years later, there are plenty of reasons why they wouldn't be driving the same car. They sold it, it's getting fixed, we (the audience) didn't see it, there could be many reasons.
Correction: Yes, it's actory Eugene Levy, who is almost always in every Steve Martin film. If you are a fan of Steve Martin films, you would recognize that and not be surprised that Levy had a role in the second film as well.