Directly following the scene where Kenny lights himself on fire and is put out by a salt truck, the boys go to the hospital scene (that mocks the show ER). Every time the camera focuses on the boys (ie, when Stan pukes and Cartman says "that is sick"), on the walls behind them you can see two posters (one says "All employees must wash their hands," the other is a chart of some sort) on either side of the doors.

After Kenny's heart explodes and blood shoots everywhere, however, the camera again goes to Cartman, Kenny, and Stan (where they say "oh my God, they killed Kenny, you bastards!"). On the walls, you will notice that the posters are no longer there!

Magic?