Other mistake: When the kids are playing "Carry On Wayward Son," there are no notes on the TV in the one area. However the boys play the notes there and get it right.
Continuity mistake: When Stan is at the arcade and stops playing, the score on screen goes back to zero without the game resetting. (00:12:00)
Continuity mistake: When Sharon is washing the dishes her right hand is on the top of a plate, camera cuts and now her hand is much lower. (00:01:05)
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.
Bishop73