Continuity mistake: In the supermarket when Kevin puts the groceries on the conveyor belt, he sets the tissue paper standing up. The next shot of Kevin shows the tissue on its side and farther back, the next shot it's back to its original position.
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 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.