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.
Visible crew/equipment: Just before Clark goes down the hill on the saucer sled you see the view down the hill. If you look carefully at the right of the screen you can see two pinkish cables stretch into the distance. These are the pyrotechnics that make the explosions Clark sets off. You can see it clearly on the DVD version, but not too clearly in the VHS one.
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.
Revealing mistake: When Uncle Louis drops the match in front of where Eddie empties the "chemical toilet" if you look VERY close you can see the rope that launches him into the air.
Revealing mistake: When Clark and Ellen are in the bed and Clark's hands are sticky, you can see that he actually grabs her hair to give the illusion that they are stuck, and he turns one of the pages of the magazine just fine.
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: The blinds are ripped out of the neighbors' window frame when Clark "harvests" his replacement tree from outside. However, when Margot is outside telling Todd to "slug that creep in the face", you can see blinds are still in the window.
Continuity mistake: When the Griswold's are driving to the tree farm, they are being harassed by the rednecks in the truck. Several shots of the front of the truck show only two passengers, but one view of the truck through its rear window shows three passengers.
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.