Simpsons, The Easter Egg - Simpson's/South Park

You see this a lot in animated shows, particularly edgy primetime shows like "The Simpsons", "Family Guy", and of course "South Park". This usually occurs because in the course of producing an animated series, captions must be submitted to the captioning services early on, and are based on the final dialogue script. Last minute changes are subsequently made to the aired dialogue due to time constraints, a director's whim, or in this case, because Fox's lawyers didn't want to upset Comedy Central's lawyers. If you watch any episode of "The Simpsons" simultaneously with captioning active, you will almost always find at least one or two lines changed, and the original line is usually funnier (and potentially more inflammatory) than its replacement.

Yeah, and this is only the simpsons' side of it. There was a whole episode of south park about the simpsons. Butters was trying to think of an evil plan, but whatever he thought of, his partner (name unknown, nickname "general dissaray") informs him that the simpsons have already done it. The other plot, in which Cartman, Kyle, Stan, and Tweak create a society of seapeople-semen hybrids, directly parodies the simpsons halloween segment "The Genesis Tub".

uh, sorry to tell you, but if you watched that episode of south park, you'd have heard that that episode of simpsons was a rip-off of another show, i think something along the lines of Twilight Zone.