Continuity mistake: When Cartman puts the bag down when outside the Chinese place, the bag moves closer to the grass between shots. (00:06:00)
Continuity mistake: When Cartman is freaking out in P.F. Chengs, we see him from Butters' view across the table, and the red bottle with a label on is facing the camera/Butters. When the camera changes to the side, the bottle also changes with no-one touching it as the bottle is now facing the side. (00:07:30)
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