Characters appear and disappear in the background while Kelly and Mac are speaking as their new neighbors move in.
During the fight with Teddy and Mac, Teddy is wearing gray underwear in the bedroom/waterbed shots, but when Teddy is coming up the stairs to his room, he is wearing white underwear.
After being thrown into the ceiling by an airbag, Mac tells Kelly that there are 3 air bags left. One is in Jimmy's seat at work which throws him into the air in the same scene, the other goes off when Mac is arguing with Kelly prior to her leaving, and the final one Mac finds by poking a seat in the house making that 4 total. After finding the 4th one, he continues to check Stella's baby crib even though there are none left.
Before Mac and Teddy start to fight. Teddy takes his hat and jacket off but after the shot of Kelly trying to get into the fuse box. Teddy takes his hat off again.
When preparing Stella for the baby calendar, when the view looks through the digital camera you can see Kelly putting in the blue ice while she had already started filling it prior.
It's a plot point that there were four airbags stolen from the car. That model (late '90s Subaru Outback) only has two, one each for driver and front passenger.
The fraternity can get arrested for theft and damage for breaking into Kelly's car to steal her airbags and also get arrested for attempted murder or assault for their airbag prank. If Mac or Kelly sat on the chair with the airbag in it holding their baby, the frat boys would definitely go to jail for assaulting a child.
Obvious stunt doubles in during the fight between Mac and Teddy.
In the opening scene, when Mac and Kelly are having sex, you can see Kelly's panties around 2:00.
When the frat boys shut down the party and make everyone leave after finding out the letter from the school was fake. Mac, Kelly and Jimmy try to sneak into Teddy's room to get to the fuse box to turn the party back on. But Scoonie is seen turning the party back on from the fuse box on the side of the house outside. They should've went to the fuse box outside instead of in Teddy's room.
When Mac and Kelly are talking to their friends on video chat on their laptop, the camera only shows the laptop. You can see Mac's reflection on the laptop screen and the mouth movements don't match what we hear.
When Mac and Kelly are talking to their friends on video chat on their laptop, the camera only shows the laptop. You can see Mac's reflection on the laptop screen and what Seth Rogen is saying off-screen doesn't match what the reflection is mouthing.
The morning after the party where they get drunk, when Kelly tries to breastfeed the baby mac stops her telling her she needs to pump and dump so she doesn't expose the baby to the alcohol in the breast-milk. Alcohol only remains in breast-milk for two hours after the last drink, and it had been several hours since Kelly's last drink and her feeding the baby. Pumping and dumping is only for comfort if you're going to be uncomfortably full in that two hour window, so it would have been perfectly safe for her to breastfeed.
The university logo on the letter Jimmy sent was said to have the Hebrew phrase "Game over" (Sof HaMisChack) but in fact it a slightly different phrase "the game has ended" (HaMisChack Nigmar) and the Hebrew writing is in reverse.
At the hospital, Mac is seen breaking character by smirking before Kelly shouts and attacks the doctor.
At the hospital, after the doctor falsely tells Mac and Kelly that their daughter has HIV from an unused condom. When Kelly starts to shout and attack him, he says "ma'am ma'am" he is heard laughing.
When Mac and Kelly see the fraternity move in to the house next door. Mac calls Pete "blonde dude with red sleeves" but Pete has brown hair.