Revealing mistake: When Clark notices that Lewis has caught on fire, he's only burning when he pushes him on the ground.
Visible crew/equipment: After the cat is electrocuted and begins smoking underneath the chair, the next camera angle still has the smoke machine in the shot next to the chair.
Continuity mistake: When the family is on the front lawn to inaugurate the Christmas lights, between shots a blue Lincoln at the curb disappears and a station wagon appears across the street.
Visible crew/equipment: When Eddie and Clark are putting the burned chair in the garbage, you can see the shadow of the camera following them.
Revealing mistake: When Julia Louis-Dreyfus has the dog jump on her, you can tell it's a stunt double. In the shot of Julia, the gem on the back of her sequence dress and her hair doesn't go below the gem. When Snot is about to jump on her, the hair completely covers the gem and is much fuller and looks like a wig.
Other mistake: When Clarke and Eddy are drinking eggnog by the tree, and Snots is drinking the tree water, look at the tree's trunk. It's a birch tree not a pine.
Continuity mistake: After Clarke falls in the bush, the gutter repairs itself.
Revealing mistake: After the squirrel lands on the coffee table, it rises up and twitches its tail in a mechanical motion, showing it is really a puppet.
Continuity mistake: As the shots quickly change from the front of Catherine and Rusty to their backs, Catherine is either holding Rusty tightly to her, loosely, or not at all.
Visible crew/equipment: Just as Clark begins his explosive argument, there is a shot of the family watching in horror, and the top of the set is visible at the top of the frame.
Visible crew/equipment: When Ellen opens the door after Clark falls from the tree, there's a strange white thing visible for a few frames at the upper left of the door. Might be the hand of a crew member opening the door.
Continuity mistake: When Aunt Bethany asks "Do you hear it?", she leans forward in her chair. In the next shot she's suddenly sitting back, and leans forward again when she says, "It's a funny squeaky sound."
Revealing mistake: When Clark and Eddie push the chair to see what happened, the burned cat is only visible in closeups. There's also a blue object next to the couch that look like an iron that is creating the smoke effect.
Continuity mistake: When Clark's neighbours return home, the man says to Clark, "Where do you think you're going to put a tree that big?" The trunk is very large, bigger then Clark's feet. Clark replies, "Bend over and I'll show you." Then Clark cuts the tree trunk - it is very thin and easy for him to cut.
Audio problem: During dinner on Christmas Eve there is a shot of the cat chewing on Christmas lights. The sound of the cat chewing is a fraction of a second off.
Factual error: Even though this movie takes place at Christmas and there is lots of snow around the outside of the house, you can tell this was filmed during the summertime due to the short shadows in some outside scenes. At Christmas time, shadows are longer all day long.
Continuity mistake: When Clark is stapling the lights to the roof, his leg is tangled in the sting of lights causing him to lose his balance and slide of the roof. As he slides/falls grabbing the gutter the string of lights around his leg disappears.
Continuity mistake: After Clark dedicates the Griswold Family Christmas tree and it expands, Ellen's hands are on her hips, but the next quick shot of her shows her hands clasped behind her back.
Factual error: The family is reacting to where Clark went flying down on his saucer, ending up in a Wal-Mart. With the speed and length of the trip, he would've been out of sight in seconds. There was no way that they could see him from where they were.
Continuity mistake: When Rocky runs out of the RV and stands in front of his mom, her mittened right hand is on his shoulder but in the next shot it's not.
Answer: They didn't go to a Christmas tree farm. He took them to a huge forest to get one most likely because it would be easier to get one free then to pay for one.
They went to a tree farm. After running off the road and jumping the snowbank the wagon crashes through a sign that says "Trees." Clark then says, "We're here...and we made good time too." The humor is Clark forgoes a normal "farmed" tree for the "wild" monster he takes home.