Revealing mistake: The family runs upstairs with the dog and the squirrel and then runs downstairs. You can see a cut in the movie, because the rug moves. (01:21:30)
Continuity mistake: After Clarke falls in the bush, the gutter repairs itself.
Continuity mistake: At the beginning of the movie, the Griswold family makes marks in the snow when they arrive at the tree which Clark wants. A shot later, the marks are bigger even though they haven't moved. (00:08:05)
Continuity mistake: When Clark is outside hanging the lights, he staples the sleeve of his shirt, under his wrist, to the eaves of the house. In the next shot, as he is pulling, the shirt is stapled over his wrist. (00:21:50)
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.
Continuity mistake: When the squirrel jumps of the Christmas tree it lands on the floor, but in the next shot it is on the coffee table. (01:19:50)
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.
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.
Continuity mistake: After Clark cuts the rope restraining the tree branches, Rusty is very close to the tree, but the final shot when Clark gives the "OK" sign shows Rusty about two feet from the branches.
Continuity mistake: After Eddie's arrival Clark is on the lawn with "Snots." Clark's left had is shown as he is there alone with the dog trying to get his attention and his glove and wedding ring have both disappeared.
Visible crew/equipment: Just as Clark is about to smash through the hut on the greased up sled, the shadow of the camera is on the hut. (00:50:57)
Continuity mistake: When the grandfather is sleeping in the top of the bunk bed, there's a cheesecake poster on the ceiling. When Clark falls through the attic floor onto the bunk bed, it's not there.
Revealing mistake: When Snots the dog chases the squirrel into the Griswolds' dining room, he barely brushes a high-back dining chair that immediately falls on its side. If you look closely, a string attached to the the right side top of the chair can be seen tightening which then pulls the chair down.
Continuity mistake: In the scene of Clark standing in the snow trying to figure out why his Christmas lights did not work, the paper he is holding in his right hand is mid torso to him, but in the next shot when Rusty leaves him, his right hand has instantly dropped to his side and he raises his paper-holding hand again.
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.