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 is sliding down the hill on the sled, several times you can see fins on the bottom to help whoever was riding it control the direction a little better. When Clark is rubbing the oil on the bottom of the sled before he starts it is completely flat on the bottom.
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 lights the cigar that sets the Christmas tree on fire, watch closely. It shows side shots of Clark eating his dinner, and you can see the tree in the living room. The camera cuts back to Lewis lighting his cigar, and then back to Clark at the exact same angle as before. The tree is gone.
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.
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.