The Aqua Teen Hunger Force episode "Boston". It was supposedly done in reaction to the Mooninite Bomb Scare of 2007.
Unlike most of the other banned episodes listed here, this one is really banned. We know absolutely nothing about the plot, it has never been run anywhere or shown at conventions, and it wasn't even released on DVD.
Zim: "I thought the Tallest killed you."
Skoodge "Yeah, but I'm all right now."
Mad Jack the Pirate: "You were DEAD! How do you get better from being DEAD, you Scottish twit?!??"