In "Kenny Dies", Cartman attempts to get stem cell research (using aborted fetuses) legalized in a feigned attempt to save Kenny's life. It is revealed at the end of the episode he really wanted to use stem cells to clone his favorite pizza restaurant.
In "Ginger Cow", Cartman actually proves my point by completely pulling a 180 when it benefits him. Initially, Cartman keeps up the prank that the ginger cow is real. When confronted with an outraged Kyle, the school, the media... Cartman holds to the claim that the cow is real.
But then, peace in the Middle East is created specifically because the ginger cow is thought to be real. Cartman suddenly feels the need to come clean, but he confides that in Kyle, specifically. It's not all that subtle that what Cartman really wants is to extort Kyle, by making Kyle his servant in exchange for not revealing that the cow is a fake.