I just have finished reading the Digital Serendipity article at The Escapist, and another example came into my mind of game glitches and unexpected player behaviours becoming something revolutionary.
I'm of course talking about the old king of FPS, Quake. Quake was one of the first true 3D games created by id Software and released on 1996, when Duke Nukem and other FPS were using 2,5D with map tricks to simulate water and multilpe heights. It was a revolution for many things, but one of them was the game engine.
You could walk, run, jump, press switches and shoot weapons. It had some physics simulating a virtual world, such as a maximum jump height, maximum running speed and being damaged by explosions from rockets and grenades.
The level designers created the four episodes and deathmatch levels (for multiplayer) based on this premises, and initially players adhered to them.
But something unexpected happened: When hit by an explosion, you would get pushed back. If hit near enough, you would go further back. So one day, someone though: "what would happen if I jump precisely at the same time that the rocket explodes in the ground?". And thus, rocket jumping was born 
You could literaly fly up to unexpected heights, that allowed you to camper on the ceiling of the first level and caugh other gamers in multiplayer without even knowing you were there.
Same would work with grenades (grenade jumping), although with a harder timing. And if you were skilled enough, you could throw a grenade and just when it was going to explode, fire a rocket and combine the explosion of both to achieve insane jumps.
But this is just half of the story.
There was another concept flying around, the speedruns. Completing a game as fast as possible one one round .
Some guys (the QDQ team) created a tool to save and load Quake status between levels, and finished the whole game in just about 19 minutes on the hardest difficulty level.
They studied the maps of every level, the behavior of each enemy, the disposition of health packs, armors and bonus (like the quad damage) and took advantage of level design flaws to finish some levels in less than a minute. They also found small exploitable engine flaws like one allowing you to do a crazy insane jump from a slope with a certain angle if correctly timed.
Due to the Quake engine design, when jumping you would maintain the "jump speed" if precisely timing another jump exactly when touching the floor. This, among some other engine "flaws" related to how straffing and turning allowed to break the maximum running speed limit, by jumping like a rabbit. Another concept born, the bunny-hopping. Players moved jumping instead of running, zig zagging and also making harder to hit by enemies.
People realized that doing a diagonal rocket jump allowed to get a huge speed boost, higher than the normal running speed. Combined with bunny-hopping, you would get insane speeds and maintain them through big parts of a level.
The QDQ team striked back, and in the end they were able to finish the game in around 12 minutes by using also the bunny-hopping technique (among rocket and grenade jumps).
This made history in FPS. When the QDQ team did a speedrun of Quake II, they were able to finish one level in 8 seconds. Imagine weeks of work designing a level and then someone using a invincibility + quad damage + rocket jump to go up and go through the exit door in 8 fucking seconds XD
In Quake 3, even the computer controlled bots do rocket jumps sometimes, and combined with plasma shot propulsions, people could almost walk walls and fly. I firmly recommend you to watch this video for a good compilation of Quake III advanced rocket jumping examples.
In the past, game glitches allowed you to achieve the impossible, to tweak and extend a game beyond its limits, to break the world rules like Neo in Matrix.
Now, bugs make your console or PC hang or destroy the game experience. But hey, instead you can get a nice "virtual medal" achievement for losing dozens of hours finishing once and again a 5 hours long game and killing 10.000 cpu-controlled enemies ;)
 Horizontal rocket jumping was known in Doom according to John Romero, but vertical one came with 3D and Quake.
 Modern PC games with mod support allow to really do it in steps, by saving the game state, health and inventory between levels, unlike speedruns of console games which are really done in one sit.