Continuity mistake: When Clark, Eddie, and Catherine are admiring the Christmas lights of the Griswold house, Clark is very close to Eddie with his left arm draped over Eddie's shoulder, but the next quick cut to Clark when he says, "Eddie?", shows Clark further away and his left arm at his side, and not stretched out.
Continuity mistake: When Clark falls over while on the ladder, after ripping his sleeve off, his hands keep changing position on the ladder. When he first starts to fall, his right hand is gripping the side and in the next shot both hands are gripping the rung. In the next shot both hands are on the side of the ladder until the shot after he pushes himself away from the tree and they go back to being on the rung for a shot.
Revealing mistake: When the SWAT team arrive at the house, a member of the team runs towards the house and dives in the snow. However, instead of falling in the snow as you would expect, the white fabric on the floor bunches up.
Revealing mistake: When Clark (on the saucer sled) reaches the bottom of the snow hill, and is on the highway, as the headlights of the truck are coming at him and the "sparks" are flying from under the sled you can see the wheels underneath to make it roll across the highway to reach Walmart's parking lot.