Continuity mistake: In the prosecuting attorney's speech in the first courtroom scene, the judge's house of cards is no more than 2 levels high (a base level with a peak) [0:2:20]. In the close-up shot of the judge, the house of cards changes to 3 levels high (2 base levels and a peak)[0:2:35]. When the scene focuses back on the prosecuting attorney, the house is back to two levels of height[0:2:39].
Deliberate mistake: When the saloon patrons pour through the swinging doors in pursuit of Packer, the same people are seen repeatedly. A relatively small number of extras are running around to the rear of the set and re-entering through the back so that they will appear to be a larger crowd of people.
Deliberate mistake: When Swan sings "Let's Build a Snowman," the snowman looks like your basic run-of-the-mill snowman in almost all of the shots. However, there is one long shot where the snowman's head is much narrower, and looks suspiciously like the alien "visitors" from South Park. [If you watch the DVD with the director's commentary, Trey Parker and others admit to putting aliens in random shots of the movie. It is during the bar fight when they say this, right as a shadow of an alien is on the wall behind the actors. This was done on purpose.]
Audio problem: The original audio track for the saloon scene towards the end was accidentally lost during filming. Because the film was largely ad-libbed, not all of the lines could be recalled when the scene was dubbed in post production. As a result, lip movement is matched rather poorly to the audio.