Cartman's Mom is a Dirty Slut (1) - S1-E14
Plot hole: Stan's grandad submits the video of Kenny being hit by the train. This gets through into the final, which shocks Stan and Kyle. But shortly the clip would have had to get through to the final by another episode, so shortly Stan and Kyle wouldn't have been so surprised. The video couldn't have been submitted before, as Stans video was submitted first.
Answer: He appears in at least 5 other episodes, but usually just as a background character. He can be seen at Cartman's party in "Immaginationland". He was also one of the kids trying out to replace Butters in the "Professor Chaos" episode.
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