If there's one constant on South Park, it's that Eric Cartman is the most selfish jerk in the entire town. It's possible that he's the most selfish, spoiled brat in the entire state of Colorado, if not the entire country. While there are innumerable examples at this point of how deep his selfishness runs, there may be no greater example than the now infamous Casa Bonita incident.
Cartman's selfishness is legendary at this point. When Stan's mom brought KFC home for the boys one night, Cartman ate all the skins off of each piece of chicken and ran away. That led to the boys ignoring Cartman at school. Instead of realizing that everyone was mad at him and changing his behavior, he assumed that he had died on the toilet and started acting like a ghost. In addition to being legendarily selfish, Cartman is also legendarily stupid.
One of Cartman's favorite places on Earth is Casa Bonita, a Mexican themed restaurant in Denver. It's an over-commercialized and cartoonish interpretation of Mexican culture that would appeal to the residents of South Park. Casa Bonita is actually based on a real restaurant of the same name in Denver that Matt Stone and Trey Parker had gone to as children.
In Season 7, Episode 11, Kyle planned a trip to the South Park version of Casa Bonita for his birthday. His mother allowed him to invite three friends. Naturally, both Stan and Kyle were on the list. But the fourth spot went to Butters, something that didn't go over well with Cartman. And as is often the case when Cartman isn't happy about a situation, he came up with a devious plan to come out on top.
Kyle didn't invite Cartman because he was tired of Eric being a jerk. So Cartman pretended to be nice, lulling Kyle into a false sense of security. Kyle finally told Cartman that if one of the other boys couldn't go, he could substitute in. Then, Cartman did the only rational thing he possibly could, and pretended that the world was ending. Under the guise of surviving a post-apocalyptic scenario, he then convinced Butters to hide in a fallout shelter.
Making matter worse, the Casa Bonita trip was delayed due to Butters going missing. Cartman then had to imprison Butters for a week until Kyle could take him on the Casa Bonita trip. He even had to move Butters before he was found, resulting in Butters getting lost in a dump and trying to rebuild society single-handedly. A woman eventually showed up at the dump and explained the truth of the situation to him.
As they entered Casa Bonita, Mrs. Broflovski got a call about Butters. Cartman picked up on what was happening so he rushed to enjoy all of the activities Casa Bonita had to offer in a few seconds before he was taken into custody. The most confusing aspect of the whole situation was the fact that Butters trusted Cartman at all. His abuse has been directed towards everyone, though Butters tends to take the brunt of it. At some point, he needs to figure out that Cartman cannot be trusted.
Rather than simply accepting Kyle's initial decision, Carman kidnapped Butters and essentially held him hostage until he got what he wanted. But for his plan to work, Butters had to trust him. He went along with everything, and it almost succeeded. The worst part was that neither boy learned anything from the experience. Cartman is still incredibly selfish and Butters is still incredibly gullible. Some things never change in South Park.