Other mistake: When Buddy's brother is reading Santa's list he reads the newscaster's wish he looks and mutters something about letters meaning they were in alphabetical order. However when he is saying random names he does not turn the page and the names are NOT in alphabetical order.
Visible crew/equipment: In the scene where Buddy and Michael are leaving the school, a crew member is visible in the bottom left of the shot. At first, he is just standing there, but he then tries to hide inside a doorway.
Continuity mistake: In the very beginning, when the nun puts the baby in the crib, she puts a blanket on him he pushes it down to his feet, then the camera goes to the nun walking out of the room, and when we see the baby again the blanket is covering him again.
Continuity mistake: When Buddy eats dinner with his dad and "family", there is a piece of bread in front of the dad that has a bite taken out of it. Throughout the scene the bread changes positions.
Continuity mistake: During the scene where the stepmom is eating spaghetti, you can see she is only using a fork and there is a knife next to the plate. In the next shot the knife is now on the plate despite the fact she only uses the fork to eat throughout the scene (also there is no sauce on the knife so very doubtful she used it to help her eat the spaghetti). In the following shot of her the knife is back by the side of her plate again.
Revealing mistake: In the scene where Jovie is taking a shower, for an instant, you can see her bathing suit top at the bottom of the screen.
Continuity mistake: The scene in James Caan's publishing conference room when Buddy and Myles Finch are wrestling results in Buddy's foot hitting the far left picture of three pictures on the wall. The picture comes crashing down and you hear breaking glass. The next shot reveals the fallen picture back up on the wall unbroken. It is followed by the next shot showing the picture gone off the wall again.
Continuity mistake: In the scene where Buddy is singing along with Jovie (who is in the shower), it shows Jovie singing and her hair is obviously wet. However, when she stops singing because she hears Buddy, the shot cuts to Buddy on the counter, then back to her. Her hair is dry and stringy on the bottom. It is more obvious when she sticks her head out of the shower.
Continuity mistake: When Buddy chugs the soda, the bottle rotates in his hand depending on the shot. In views from two different angles, the Coke logo is always visible.
Continuity mistake: In the scene when Jovie is asking Buddy why he was in the girls locker room, the amount of people in the background changes each time it cuts back from Buddy to Jovie. It goes from originally five people to three, back to five and so on. This is obviously a mistake because the characters in the background are the same when it's five and the same when it's three.
Continuity mistake: In the scene where Buddy's Dad is talking to his manager about the missing pages in the book, and how the manager thinks that the company will post a minus eight for the quarter and storms out of the room, Buddy's Dad's arm is resting on a piece of paper that is almost directly in the center of the table, and after it cuts to the manager and back to Buddy's Dad, the piece of paper is close to falling off the table.
Revealing mistake: When Buddy is chugging the bottle of Coke, there is a shot taken from behind Buddy. Look at his arm when this is happening. The dark line running down his arm is the tube used to carry some of the Coke away, so Will Ferrell didn't have to drink the entire 2 liters.
Revealing mistake: In the beginning of the movie, there is a scene where Buddy hits his head on the roof of the house as he's running out. You can obviously see a machine pushed the snow forward to land on his head. [May be easier to see in fullscreen version].
Continuity mistake: The piece of silverware in the middle of the mobile during the spaghetti dinner hangs at different angles in different shots.
Continuity mistake: In the scene where Buddy has made breakfast for the family, his "step-mom" eats the spaghetti taking from the side of the plate and does not touch the top. When the camera goes back to the plate the spaghetti has been all stirred up with the sauce now mixed in, instead of just piled on top.
Revealing mistake: Towards the beginning, in one of the scenes where Buddy is sitting on Papa-Elf's lap, you can see Bob Newhart is not quite lined up with Buddy, or his legs in the foreground (Which are actually the legs of a little kid), as he is really a short distance away, to make it look as though he is shorter.