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 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.
Obvious stunt doubles in during the fight between Mac and Teddy.
Characters appear and disappear in the background while Kelly and Mac are speaking as their new neighbors move in.
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.
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.
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.
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.
In the fight scene between Teddy and Mac, Teddy's hair is perfect when he removes his skullcap and starts dancing around with his fists. But after one groin kick his hair is mussed and he's sweaty as if it's his thirtieth take.
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.
After Mac and Kelly finish talking to Teddy about keeping the noise down for the first time, they start walking away from the porch. The next shot shows Teddy talking to Pete for around a minute. Teddy then opens the door to see if they want to join their party, but they have only traveled about halfway down the path that leads to the porch. Mac and Kelly should have traveled farther away from the time it took Teddy to talk to Pete.
When trying to convince "ass juice" to help them he picks up his milkshake and takes a drink, you see a cherry as Kelly scoops up some whip cream on her finger, yet when "ass juice" sets the milkshake back down the cherry is gone.
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.