Revealing mistake: Whenever Marv is outside walking in the snow in his bare feet you can tell that the feet aren't real.
Continuity mistake: As Marv loses his last sock on the tarred stairs, his heel does not make contact with the tar. Later, when he picks up the crowbar to walk out, his whole foot is now covered in tar.
Continuity mistake: The amount of tar on Marv's feet changes from all over the bottom and sides, to just the bottom to almost none. Even though he walked through the snow, the tar would not just wash off.
Revealing mistake: When Harry tells Marv that he's sick of him leaving the water running in the house that they robbed, you can see Harry supposedly shift his van into drive. However, Harry does not even move the gear selector from park to drive.
Continuity mistake: When Kevin puts down his milk and snack on the table, he is looking at the presents under the tree. When he wakes up thinking his mom might be home and calling for her, when he goes in the room where the tree is, there are no presents.
Factual error: Plane doors are only closed when the flight is ready to take off and most passengers have been accounted for. They are not reopened for late passengers, so in reality the family would have been left behind.
Revealing mistake: When Marv is trying to break into the basement, after he slides down the stairs and is trying to use the crowbar to hoist himself back up, the "ice" on the floor looks "too perfect" and is either a greased up glass sheet or plastic (to allow him to slide his feet around comically). You'd sort of expect the ice to be a bit uneven naturally. It's more evident in HD or digital versions.
Other mistake: When Gus Polinski talks about his music, he mispronounces "Daj mi buzi" (Die-Me-Boo-Gee) as "Domavougi." "Daj mi buzi" is Polish for "Give Me a Kiss."
Suggested correction: I think this is meant to be a "deliberate mistake ", which didn't show up as an option to suggest making the change elsewhere (under "edit"). It appears to be part of the comedy - he should know how to pronounce it. Another opinion would be helpful.
Continuity mistake: Kevin's hands change during the micro-machines sequence. He is sitting at the top of the steps, and puts his right hand down. In the next closer shot, both hands are in a different position.
Continuity mistake: After Kevin's whole family comes home, he walks straight to the window to look out. If he had done that, he would be looking out of the side of the house, but he is obviously looking out the front of the house to straight across the street.
Continuity mistake: When the delivery guy stands outside the door and Kevin plays the movie, in the shot from Kevin you see the red curtains are more wide open and you see the silhouette from the delivery guy. In the shot from the delivery guy the red curtains are suddenly more closed and his silhouette couldn't be showing through.
Revealing mistake: When old man Marley hits Harry and Marv with the shovel at the end, you can see it's actually a stunt man doing the hitting motions.
Continuity mistake: Buzz comes down the stairs 3 seconds before Kevin, but manages to eat Kevin's entire cheese pizza in the next shot before Kevin could enter the kitchen. (00:08:04 - 00:09:20)