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: 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 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 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.