Remember Super Mario World, for the SNES?
Besides the story and level outline put forth by the game, at its heart exists a very nice game engine. As such, Japanese hackers have had at it, with mixed results. Some hacks were great, don't get me wrong. But some hacks were...
Kaizo Mario World
Kaizo Mario World, often referred to as 'Kaizo Mario', for short, is an absurdly difficult video game. It is commonly "translated" as Asshole Mario World, but this is due to a misunderstanding by the original poster to the internet: 'Kaizo' actually means 'hack', and the original title was something along the lines of "Making my friend play through my Mario (Super Mario World) hack" (allegedly from the Japanese '自作の改造マリオ(スーパーマリオワールド)を友人にプレイさせる'). Regardless, the name Asshole Mario certainly fits as a title for this torturous game.
The characters and enemies are the same as in SMW. The levels, however, are dangerously different. The first moment of the first stage finds Mario stuck behind a five-high wall of Bullet Bill Cannons, and just barely enough run-up and time to escape that situation. Right afterwards there is an enormous gap that requires you to ride the five-high wall of Bullet Bills across. And the madness only grows from there on.
Invisible blocks are used to great effect, especially in those moments where you needed it to not be there the most. Things you never thought Mario could do, like jumping only three blocks high to avoid a low-hanging ceiling of death, or catching a parachuting Goomba in mid-air to bounce yourself just that extra inch further and survive, suddenly need to not only be possible but consistently performable. The number of deaths Mario dies will only reach the triple digits... if you're lucky. Most emulators come with a savestate function, allowing you make an instant makeshift savepoint to come back to as many times as you want: this ability becomes your bestest friend in the whole wide world. There are enough levels that actually completing the game is doable, though not without hours upon hours of practicing and failed attempts.
Of course, leave it to the internet to take something and run with it.
As SMW hacking became more widespread, a certain affection developed for hacks of this mind-bendingly evil caliber. Hacks of this difficulty level became known as Kaizo Hacks, and groups of people formed that would come together and share such difficult hacks to challenge each other and themselves, both in gaming skill and in level designing ability. SMWCentral, a site dedicated to hacking Super Mario World, has (somewhat) recently opened up a separate forum for Kaizo hackers and players, as they don't normally fit the site's "good design" criterion of being completable by normal human beings.
MoltovMarioWorld, an avid hacker and player at the time, really took to the idea of Kaizo hacks, and one day provided the internet with the natural extension to the idea: Pit hacks. Beginning with 'Pit of Death', these hacks involved literally frame perfect timing, with death as a consequence for any wrong move. Their intended use is solely for TASing: normal play is completely impossible. The appeal is only to those masochistic enough to huddle over a computer screen for days on end, the same 3 second loop of music repeating forever in their heads. Videos of the playback, however, end up looking like some sort of magic.