Continuity mistake: When Uncle Lewis and Aunt Bethany have just arrived and Aunt Bethany says, "Oh dear, did I break wind?" Ellen helps Aunt Bethany fully remove her coat. Seconds later, within the same scene, Ellen is still helping Bethany off with her coat.
Visible crew/equipment: When Clark is on his sled, just before he crashes into the cabin, the shadow of the camera is visible on it.
Continuity mistake: After the tree burns up from Uncle Lewis' stogie, he turns around revealing fire on his back. Right as he is turning, the shot cuts to Clark and back to Lewis. In this new shot, Lewis' hair is different (compare the wigs), indicating the person getting the fire on his back put out is a stunt person.
Continuity mistake: When Uncle Lewis and Aunt Bethany arrive, the wreath on the front door is lit with Christmas lights. When the door opens from inside the home, there are no lights on the wreath....it is just plain green.
Revealing mistake: When the neighbors fall onto the floor of their bedroom when Clark's Christmas lights finally go on, you can tell a stunt double is standing in for the female neighbor as she falls down at the foot of the bed.
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.
Continuity mistake: When Clark is going crazy, he cuts the baluster. But it's still in place the scene later, when the family have problems with the squirrel.
Continuity mistake: After Eddie et al arrive at the Griswolds, "Snots" is introduced. When everyone else has gone into the house, Clark is left outside with the dog. A shot shows the dog at Clark's side, and if you look at Clark's sleeve, he's wearing a sweater, but actually has on a windbreaker. At the end of the movie, Clark's wearing the sweater. Apparently the shot was taken of him and the dog, and edited back into the film early on.
Continuity mistake: When Clark's uncle drops the match that sets the sewer on fire, his toupee falls off and at the end of the scene you can see him trying to put it back on, although it's burnt pretty badly. In the final scene where everyone is dancing around, his toupee is back on and in perfect condition.
Continuity mistake: After Clark receives what he thinks is his bonus check, he says if there is enough money left over he will fly all of the relatives out to see the new pool. Everyone reacts with excitement, particularly Ellen, who is standing right next to Clark. She lets out a cry of excitement and raises her arms. The next shot is taken from the opposite point of view (from over Clark's shoulder). In this next shot, you can see everyone else still excited with the sudden exception of Ellen. She went from shrieking in excitement to having her mouth closed and her arms back down at her sides.