When Joel (Jason Bateman) comes home from being punched at the bong session, he has a monster of a black eye. When he leaves the house the next morning, the shiner has completely vanished. (It reappears briefly, but in milder form, by the time he arrives at work.)
In a scene involving Jason Bateman and Ben Affleck at a table drinking beer, there is a glass of water near Ben's beer bottle that disappears during the scene.
In the cemetery scene, Suzie's hair changes position several times as shots change, between pulled back over her shoulder to hanging down in front.
The collar of the gigolo's sweatshirt changes within the same scene when he is talking to Jason Bateman and Ben Affleck about being hired.
While Joel and Dean are drinking beers together at a table, the facing of the beer in front of Dean keeps changing between shots.
In the opening scene, the music store employees call the pickups on the Gibson guitar "humbuckings." Those type of pickups are either called "humbuckers" or "humbucking pickups," but not "humbuckings." Someone picky enough to fuss about the difference between "color" and "finish" would likely be the same way about pickups.
Early in the movie, when Cindy is sorting through her various forms of ID, an Ohio driver's license is shown. The issue date is April of 2001, with an expiration in April of 2010. An Ohio license must be renewed every four years.
In close up shots of Joel's car, the windows aren't tinted, but for a far away shot, they have dark black tint.
When Cindy is reading the newspaper about Step you can see that the paper was printing using a computer printer.
On the promotion poster Joel's LEFT eye is black. In the movie it's his RIGHT eye that gets smashed.
Boom microphones clearly visible in several early house interior scenes.
Dominos sells Coke products, not Pepsi.