Sometimes there's a line on the screen separating the two Cartmans, sometimes not. (This may be a reference to poor special effects used in sci-fi movies featuring multiple characters played by the same actor. Also note the badly fitting background image in those scenes.)
From time to time, various items, such as the light switch and the alarm clock, disappear and reappear in Stan's bedroom.
When Sharon Marsh asks Stan if he likes his fish she does not move her mouth at all.
Evil Cartman mentions the Wampanoag, but these are native to Rhode Island and Massachusetts, not Colorado.
Aunt Flo describes Shelly's present as a surround sound home theatre though it only has two speakers (stereo), not surround sound's five speaker setup.