If you lurk in the corners of the many fandoms revolving around cartoons, (which I really don't recommend) you are likely to find that each of them has some sort of "lost" episode attached to it. There's the ones that are outright banned, such as the infamous Pokemon seizure episode, and occasionally the episodes that weren't ever meant for airing, so the rumors say, but were instead made to distract the network from other things they wanted to sneak by and/or for the crew's own private enjoyment. These are usually just urban legends, like the supposed Spongebob episode where he dies, because realistically who would want to put the effort into making something nobody is ever going to see, especially back in the days before the internet.
One of the more infamous ones out there is an episode of the Cartoon Network show Dexter's Laboratory. Called "Dexter's Rude Removal," this episode was supposedly a super raunchy 10 minutes created by the crew with absolutely no intention of it ever getting on air. The most often cited source for this supposed episode was this forum post at Big Cartoon Forum by user TyphoidTimmy. According to him, Dexter's Lab creator Genndy Tartakovsy brought a copy of the short to Comic Con years ago, where he showed it after making sure everybody 18 and younger left. It's just THAT crude.
