At the abortion clinic, while the doctor is talking to the patient about stem cell research, the blanket over the patient briefly turns from white to the color of the doctor's and patient's robes.
The flag is displayed the wrong way when Cartman speaks before Congress. The blue field should have been at upper left. This is the way to display flags, whether they be displayed horizontally or vertically.
It makes no sense for Cartman to be pedaling his tricycle all the way up into the mountains, let alone just for fun and singing while he does, and even less sense for him to scale down into the ravine. He has always been characterized as an obese and lazy child that avoids physical exertion as much as humanly possible.