When Mollie and James are having an argument in the hallway in front of their apartment, the door to their apartment is sometimes ajar, sometimes wide open.
The Pee-Wee Herman doll on Mikey's shelf is standing/lying/sitting between shots.
In Look Who's Talking, Mikey had straight blonde hair. In this movie his hair changes to brunette and curly. In Look Who's Talking Now, it goes back to straight blonde hair.
The shaving cream on James' face disappears when he picks up Julie.
When Mikey is under the desk at Molly's office, Rona's shoe alternates from being on and off her foot between shots. However, since her foot is out of the camera view, Mikey could have put her shoe back on out of shot.
Stuart leave the apartment to take the subway into town. Molly says, "I don't want you to take the subway. Take this for the subway. There are a lot of weirdos out there, take a cab." She just said don't take the subway then says take the subway, then says take a cab.
When Rona meets Stuart for the first time she says, "Molly never mentioned having a brother." Earlier in the movie, while walking together through the city, Molly has a conversation with Rona about Stuart.