Continuity mistake: When the squirrel jumps out of the tree and the whole family is hiding around the corner in the front entry, the camera alternates shots frequently from the front of the family to the back of the family. Each time the angle changes, Catherine's stance changes from her arm being around Russ to her arm at her side.
Continuity mistake: Less than a day after the ice breaks the neighbors' window, it has fixed itself in the outside shots as Clark is stuck in the attic.

Continuity mistake: When Clarke is installing the Christmas lights, the rope on the ladder keeps appearing and disappearing. (00:23:00)
Continuity mistake: The retractable attic staircase is a type held closed only by light spring pressure, not a latch, but it doesn't budge when Clark jumps on it. When he initially opened it, and later when his wife does the same, it opens normally with a light tug from below on the cord.
Continuity mistake: When the squirrel is being chased by Eddie's dog, it passes through the broken door and then vanishes. (01:22:07)
Continuity mistake: When Clark falls off the roof, the staple gun goes down before him. In the next shot, it's not heading down from his point of view.
Continuity mistake: When they first step up to the tree in the forest, Audrey is still moving. In the overhead shot, she stops, and we can see tracks in the snow where all four have been standing. The problem is, there are a lot of tracks. We're lead to believe they just walked up to the tree.
Continuity mistake: After the SWAT team busts into the Griswold's home and an officer yells "Freeze!", Clark's right arm changes position from either being raised or lowered as the scene quick cuts.
Continuity mistake: After Mr. Shirley tells Clark he will get a bonus plus twenty percent, Catherine, Rocky, and Ruby Sue cheer and get up from the couch to mingle with the relatives while Rusty and Audrey respond excitedly. There is a close up reaction shot of Clark as he begins falling, then a long shot of Clark falling to the floor as Catherine, Rocky, Ruby Sue, Audrey and Rusty are in their original positions on the couch, calmly watching Clark fall.

Continuity mistake: When Todd, the snob neighbor, falls down the stairs, his wife Margot is not visible in the bedroom. (00:38:55)
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.
Continuity mistake: When Eddie and Clark are drinking eggnog, Eddie knocks down the decoration with propellers on it. When Eddie walks to the table with the nuts on it the camera shots cut from Eddie to Clark attempting to fix the decoration. In each shot that we go back to Clark, the number of propellers left on the decoration change even after Clark has given up fixing it. (00:45:55)

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. (01:17:20 - 01:21:55)
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: 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.

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)
Suggested correction: The position looks the same if you consider hand movement. Look at the opening of the sleeve and the hand position for comparison. I don't think this is an error.
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)
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."
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.