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 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: 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.
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: When Clark and Eddie are standing next to the Christmas tree drinking eggnog, when Eddie walks over to the decoration and knocks it over, in one shot his glass is less than half full and in the next it is almost completely full and he takes a gulp.
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: In an early shot of the outside of the house, the motorhome that Eddie and family show up in is parked in the driveway. The problem is, they don't appear until later in the film.
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: 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.
Revealing mistake: When Clark is putting Christmas lights on the top of his house, the ladder falls but stays at the same angle.
Factual error: Prior to the cat being electrocuted, Clark is seen plugging the lights into a 115 volt outlet. However, when Rusty resets the breaker, it is a double pull breaker that is reset, which typically provides 230 volts.