Continuity mistake: After Marv falls out the window during the kidnap rehearsal, the bottom part of the window fell out along with Marv. However, after Kevin walks in, there is an extra pane left on the right side of the window. And after that, when the camera switches to the outside, the window is all crooked and bent, but it wasn't in the previous scenes.
Continuity mistake: In the flood scene when Natalie walks in and begins to scream, Kevin is standing on the first step of the stairs with his shirt uneven, and the strings for his pants are showing. In the next shot, Kevins clothes are neatly ironed, his shirt is even, and the strings have disappeared.
Continuity mistake: After Natalie arrives back at the house to find her mansion has been flooded, she screams and says "what?" six times. At that time, there is a shot of a purple shampoo bottle and a rubber ducky just in front of the stairs. Then Kevin says "I know it looks bad but." and during that line you can see that the shampoo bottle and rubber duck is about 10 meters from where it was before. It couldn't have floated that far in a few seconds.
Continuity mistake: On Christmas morning, Kevin fiddles with the presents. When his father says he has to go and pick up the royal family, look at the presents over Kevin's shoulder. The view changes so it is by the Christmas tree, and the same presents are in a different position (especially the dark metallic green one and stripy one).
Continuity mistake: When Kevin escapes from the wine cellar using the dumbwaiter, he goes up. Then, he sends it down onto the robber's head. Then, the female sends it up, and then back down again when it hits the male robber's hand. But, when Prescot goes to the dumbwaiter chute, the dumbwaiter is up instead of down.