Continuity mistake: After Mr Garrison announces Kathie-Lee will be presenting an award for Cartman, you see the next scene in the Mayors office. There we can see cleary the curtains are red, then she looks out the window and looking in the office from outside, they are pink, but it goes in the office again and now they are back to red.
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Revealing mistake: Right after Wendy tries to convince the crowd that Cartman's actual paper was 'Walden' with Cartman's name on it, Stan mentions something about Kyle's mom making tuna fish sandwiches. When he does this, you can kind of see him 'lift' off the background.
Continuity mistake: When Wendy finds her report paper, it has "good effort" and her grade written in red marker pen on it, with a short note underneath it also written in red pen. In the next shot the "good effort" and grade number are still on the paper but the note underneath has vanished.