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.
Revealing mistake: When Julia Louis-Dreyfus is has the dog jump on her, you can tell it's a stunt double. In the shot of Julia, you can see 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.