Watch the Southpark Episode when Evil Cartman comes from the other dimension
In the part where Cartman ripped off Evil Cartman's Mustache and they are standing Parellel to each other, You can slightly see a dividing line in which the editors copied.
Heh.. This is NOT a slip up. If you remember watching old 70's and 80's TV series, the way they did "twins" being on the same shot at the same time, was to split screen and put the film together later. The line in the middle is on purpose, a little throwback to those film days. :)
This was totally intentional. It was made to be incredibly obvious because it is a huge joke on the way they used to do phony twins shots in old tv shows. Also, think about it, why would they need to copy half the shot, it is all done with computers, they could just hit copy and paste and add a mustache.
its a joke dude. if it were THAT obvious, then it wouldnt really be a slipup. also, its a CARTOON, so it wouldnt make any sense whatsoever if it were a slipup. you think that there are actors playing the boys and then they just go over it with cartoon graphics?