Some of the characters talk about seeing Spider-Man 3, which came out in theaters four weeks before Knocked Up entered theaters. Later in the movie, Allison interviews celebrities at the 2006 MTV Movie Awards, which took place a year before the movie actually came out.
Debbie pulls up a map of sex offenders, and talks about how they are surrounded. The film takes place in California, but the map shows Boston (Interstate 95 and 93 intersect with routes 128, 1, and 3).
When Alison is sitting on the couch next to Ben as his friends leave to go play paint-ball, she has a noticeable pregnant belly. In the next scene, she is sitting on the side of the pool in a bikini with her sister and nieces, and has a flat stomach. In the next scene, the belly is back. (The fake belly couldn't get wet, this is mentioned in the Director's Commentary.)
When Ben is revealing to the family that Allison is pregnant, Allison is drinking out of a light blue coffee cup. In the next shot, Allison and Debbie are talking about Ben while he plays with the kids, Allison is holding a red cup and Debbie has the blue cup.
When Alison tells Debbie that she will be appearing in front of the camera, Debbie tells Pete to pay attention. At first, Pete's headphones are on backwards (cables on the front of the ear-cups); as soon as the angle changes to behind Pete, they are on the correct way and remain so.
When Alison is driving her nieces to school, Sadie begins to tell her about how she Googled 'murder.' As she is telling the story the seatbelt jumps from being on the side of her shoulder to on top of it (next to her neck) between shots.
In the scene where Ben and Alison are drinking and dancing the night they meet at the club, her hair alternates between neatly up and down. When they decide to leave the club, her hair is neatly tied back, but when they exit the club, it's down.
When Ben proposes to Alison, she is wearing a striped green shirt and it is night time. The next scene she is in bed eating ice cream with her sister. Then the scene after that she is in the car with her sister, it is morning and she is wearing the same shirt.
When Ben is walking up to the door for Debbie and Pete's oldest daughter's birthday party, he stops to knock. However, the door is shiny enough that you can see the reflection of Ben and a crew member walking up beside him with the camera.
When Ben is watching Cheaper by the Dozen on the TV, the film skips from a scene near the beginning (where Tom is getting all the kids up for school) to a scene near the middle (where Tom is putting a band aid on one of the girls and fishes an athletics cup out of the pasta).
Ben claims that he was so drunk that he doesn't remember the night with Alison and says that he wishes he did. Yet later he and Alison argue over the fact that he didn't use a condom, and Ben clearly seems to remember being told to "just do it already."
When Debbie and Allison are looking at the tests, Debbie picks one up and starts freaking out thinking it was the one she took because it says positive. The test she is looking at is actually backwards (result window toward the camera not towards her) so she wouldn't be able to see the results.
The goof item below may give away important plot points.
When Debbie is going through the house of her husband's suspected "lover", her hair is very curly. She catches him at the fantasy baseball draft, and they go outside. When they're outside, her hair is almost straight.