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. (00:12:30)
Revealing mistake: When Clark is putting Christmas lights on the top of his house, the ladder falls but stays at the same angle. (00:20:50)
Revealing mistake: When Clark starts to set up his Christmas lights, there is a scene where the ladder collapses. If you look closely you can see the wire coming out from under his jacket to keep him from falling forward. (00:20:55)
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)
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)
Revealing mistake: At one point when Clark is flying down the hill through snow drifts on his saucer sled after greasing it up you can see the guide wire pulling the sled. (00:50:45)
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)
Revealing mistake: After Eddie kidnaps Clark's boss, he drives off and if you look at the ground the snow looks like soap bubbles, because you see them waving around. (01:23:36)
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. (01:32:05)
Revealing mistake: When the Santa Sleigh is launched in the air by the ignited sewage, the sleigh flies over the moon. Now on the DVD version of the film, it's not clearly visible, but on the VHS version, you can clearly see there's a box around the sleigh that looks like the sleigh is under a microscope slide. (01:32:50)
Revealing mistake: After Clark cuts the rope on the large tree, it spreads out and branches start breaking windows. One of the branches has a pole of some type pushing it through the window.
Revealing mistake: When Clark is on the sled, there's one frontal shot when the snow flies in the air before he touches it.
Revealing mistake: When the neighbors fall onto the floor of their bedroom when Clark's Christmas lights finally go on, you can tell a stunt double is standing in for the female neighbor as she falls down at the foot of the bed.
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.
Revealing mistake: When Lewis's coat catches on fire after the Christmas tree explodes, the square on his coat that holds the "fire" is evident.
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.
Revealing mistake: The stunt double for Nicholas Guest is obvious after Julia-Louis Dreyfus punches him. The hair style of the stunt double is different from Nicholas Guest's hair, which is heavily slicked back.
Revealing mistake: When Clark notices that Lewis has caught on fire, he's only burning when he pushes him on the ground.
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.
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.
Chosen answer: Because there was cat food in her jello mould. He's pointing out the absurdity of someone adding cat food to jello.