Coon vs. Coon and Friends - S14-E13
Deliberate mistake: S14 Ep13 Coon vs. Coon & Friends: in the South Park universe, Canadians are depicted as looking the same (beady eyes, stick legs, heads sliced in half). Since Justin Bieber is Canadian, he should look similar to Terrance and Philip instead of having a normal face and body.
Suggested correction: I don't see how this is a mistake, deliberate or otherwise. While the creators of the show have established that to be the appearance for Canadian characters, they are well within their rights to invoke artistic license whenever they see fit. Is it also a mistake that Mel Gibson and Saddam Hussein have photorealistic faces when the majority of other characters have been established as having cartoon faces?
Revealing mistake: When Kenny is at Mick's Lanes, his eyes disappear as he pounds his hands on the table and screams through his parka.
Continuity mistake: When Stan and the user play Yahtzee, the dice is facing parallel to the cup. Then the dice faces perpendicular to the cup, then back to parallel. (00:14:40)
