Revealing mistake: When Clark is outside at night rigging up the lights, it seems as though the moon is a little too large in the sky. (00:29:35)
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.
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 Todd, the snob neighbor, falls down the stairs, his wife Margot is not visible in the bedroom. (00:38:55)
Revealing mistake: When Clark falls from the top of his house, he grabs the gutter. It bends and a icicle is thrown from it to his neighbor's house. You can see the thread which guides it. (00:24:27)
Factual error: Prior to the cat being electrocuted, Clark is seen plugging the lights into a 110 volt outlet. However, when Rusty resets the breaker, it is a double pull breaker that is reset, which typically provides 220 volts, and the breaker is off instead of tripped. (01:09:00)
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.
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: 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 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.
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: 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)
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: 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: 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 has the entire family outside to witness the lighting of the house, the positions of the in-laws change instantly from being clumped together to strung out in a line, depending on whether the shot is overhead or at ground level.
Chosen answer: Because there was cat food in her jello mould. He's pointing out the absurdity of someone adding cat food to jello.