Continuity mistake: When Brian is giving his speech at the end, it shows two policemen. The one on the left has a brown drink and the one on the right has a yellow drink. But in the next shot, the one on the right has a yellow drink and the one on the left has a clear drink.
Continuity mistake: When Peter is convincing Joe to do something with his life, they are listening to the news and beer cans are everywhere. Then the news reporter says, "Better watch out for that skateboard." After he slips, look around the room: there are no beer cans anymore.