Plot hole: When cousin Eddie kidnaps Mr. Shirley, Shirley's wife immediately calls the police. She describes Eddie as 'brutish and huge' or something similar, and wearing a 'blue leisure suit'. She also mentions that she got the license plate number of the vehicle. But how did the SWAT Team know to go to the Griswold's house to find Mr. Shirley? Eddie's RV is not registered in the Griswold's name so the plate numbers would not be much help in finding the culprit.
Continuity mistake: During Clark's rant, the shot goes from seeing him from behind the whole family to a frontal view of the family looking at him in shock. In the first, they are standing in line with the entrance to the room. You can even see Eddie's shadow on the wall as he's very close and slightly behind it. Also, on the other side, Audrey's arms are going down to her side and she's a little behind Art. In the next shot, there is no wall and Audrey is in front of Art. Her arms are also crossed.
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.
Other mistake: When the lights don't work, Clark punches Santa and knocks the antlers off the reindeer. When the lights come on and the family comes out, the antlers are back on their heads.
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.
False. They may have run over the tree farm sign, but they absolutely did not harvest a tree from the tree farm.