At the beginning whenever the Queef sisters first come on if you compare the show on Cartmans tv to when Terrance and Phillip are watching the background on the show is different.

They make character mistakes all the time. Their there one second then gone the next

Yeah, I saw that too. And Craigs shirt changes too whenever he's outside the School Counselors office but he's wearing a different shirt at lunch when they all leave.
Plus in the beginning Kyles door closed from top to bottom