Continuity mistake: The cake Howard's mom has in the car is different from the cake at the reception when the ladies are talking.
Continuity mistake: When Kevin Kline goes to school the day after the awards show, the press chase him to one of the school doors. When the scene is shot from outside, the door opens into the building and he is pushing it open, but immediately after when the shot is from inside of the school and he is closing the door, it opens outwards and he is pulling it closed.
Continuity mistake: Right before Emily and Cameron dance in the street there is a shot of the front of the bar. The cars in front of the bar are different from the rest of the scene.
Continuity mistake: After Howard crashes on his bike and he is sitting down talking to Peter there is a big wet spot on his left knee in one shot that is gone in the next.