The South Park episode, titled "Insheeption," and a CollegeHumor video both featured the same lines that mocked the complexity of the film, but were not actually from the film. One line in particular —"Sometimes my thoughts of my dead wife manifest themselves as trains." — was exactly the same.
Matt Stone, unable to obtain the actual film, pulled the line from the CollegeHumor video thinking it was from the Christopher Nolan drama.
"It's just because we do the show in six days, and we're stupid and we just threw it together," Stone told the New York Times. "But in the end, there are some lines that we had to call and apologize for."
If South Park had copied from Inception, that would have been deemed protected parody (see Mad Magazine). But if South Park copies a line from another maker of parody (a competitor of South Park, as it were), that could be copyright infringement.
The similarities between the Gurewitch/Young video on CollegeHumor.com and the recent scene from "South Park" are beyond striking, even in a blogosphere of countless "Inception" parodies. To "South Park's" credit, Parker and Stone didn't try to dodge the issue, deny responsibility or claim Incredibly Eerie Coincidence. Instead, they did the straight-up thing: They apologized.
(The repentant "South Park" creators surely are smart enough also to know they're now on a kind of pop-culture probation; were a second ripoff to occur, inadvertent or no, viewers wouldn't be so forgiving.)