When Jeff is testing the game in his room right before Alex calls to ask if he can do his levels, the action on his computer screen doesn't match up with the "Hi, You're Dead, Byebye" as the figures keep moving and fighting. As he is on the phone and his hands are off the controller, the characters keep moving anyways.
When J.P. claims Alex's game as his own and Alex argues that he created it, Mr Cheezle asks if Alex has another copy of the game. Alex says he doesn't. When a challenge is proposed, J.P. states that Alex probably stole the game and made a copy to learn all the moves. If this happened, Alex would have another copy to prove it was his anyway.
When Dante is talking to Alex about teaching his monkey Tae Kwon Do, the native Dr. says in French, "le singe va lui apprendre judo!" (which means "the monkey will teach him judo!"). Judo and Tae Kwon Do are two completely different martial arts.
Kane goes to JP and tells him the "Dwarves in level 4 are too similar to the Elves of level 6". When Samantha was giving out levels Kane was assigned levels 1-3 and Berry was the one assigned levels 4-6.
In the beginning when Alex is getting evicted the man packing in the kitchen is clearly seen placing the coffee pot into a box, and in the next shot he picks the coffee pot up off the counter and puts it into the box again.
When Alex originally flips the tray of food after removing it from the oven, he spills the food all over the oven door. The camera pans out to him in pain and goes back to the oven and the oven door is spotless.
When Jeff is sitting in the kitchen listening to Grace talking about 'giving Charlie Chaplin a hand-job', the punk chick with the foot-high Mohawk is walking by in the background with her boyfriend in netted arm-stockings. When the camera pans back to Jeff, they can be seen walking by almost in the same spot again.
During the staff meeting scene at the beginning of the movie, Samantha hands out assignments and gives Kane levels 1-3. Later in the movie when Kane talks to J.P. about a possible change he says the characters are in levels 4 and 6. These are not the levels he was given to do.
When JP is talking to Alex at Alex's grandmother's house, JP is crying. He begins to have drool hanging from his mouth. In the very next shot (close-up of JP), the drool is gone and he begins to "start" drooling.
In a scene where Kane is sitting in the game room there is a Pepsi can sitting on the table. The scene goes from Kane touching the can one moment and it being far from him the next. It goes back and forth several times like this.
Grandma and her roommates don't discover 'Antiques Roadshow' until after Alex hooks up the cable box. This implies that it is a cable TV show. In reality the show is broadcast on PBS. Having only 5-6 channels, including PBS, they surely would have already known about the show.
When Alex reaches into the oven, he picks up the hot pan with his fingers and thumbs. In the next scene, when his hands are bandaged, his fingertips aren't bandaged at all. There wouldn't really be any reason to bandage his palms, considering they never touched the hot pan.
When Alex's grandma first takes him to his room, Alex puts his
bag with the X-Box in it on his bed. The scene then changes and the bag is now on top of the TV and Alex's laptop is on his bed instead. A deleted scene (found on the DVD) takes place between these events, where Alex is looking for the proper A/V inputs on the television and puts the X-Box on top of the TV.
The goof item below may give away important plot points.
Near the end of the movie JP steals Alex's game 'Demonic' and tries to claim it as his own. After 3 years of being the sole programmer of a game on his own personal computer, as Alex claimed to have done, he would've had considerable evidence to prove he was the original programmer.