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.
Other mistake: At the Children's Therapy Center, a girl says she has a tick that makes her bend her neck and snap her fingers. In this shot, Thomas and the girl sitting next to him are reflected in the mirror behind her, but their reflections are not backwards like they should be. This is shown a few more times in following shots. (00:07:57)
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