unexpectedly, my role-playing buddies and I had discovered ZOMBIEPOCALYPSE. Except it wasn't really ZOMBIEPOCALYPSE, not yet. Back then, it was just Dungeons & Dragons (tm) with zombie-flair. But from such small beginnings...
After our initial foray into the world of the ravenous undead, our gang returned to our regular hacking-n-slashing; raiding dungeons, killing evil wizards, that sort of thing. It was as if the great Zombie attack had never happened (in fact, because that adventure had been so deadly and so many player characters died, we all unanimously agreed afterwards that it didn't really happen and the official story of our campaign resumed from just before the zombies emerged). But regardless of the canonicity of the adventure, we all agreed it had been a great deal of fun.
So much fun, in fact, that we did it again the next Halloween. And the one after that, and then again after that, and so on. "The Zombie Game" became a yearly tradition for us; a one-off unofficial adventure where we all had fun slaughtering zombies - and as much fun as we watched them slay our characters.
But as time passed, our yearly Zombie Game became increasingly distant from our main campaign. First to go were our characters, as the players took the opportunity to use create (and lose) all new heroes. Next, the setting shifted from the magical realms of D&D to the more familiar haunts of our own world. And finally, the very rules that made up the game began to change. Because our characters died so quickly (and D&D character creation was so tedious), we started by simplifying that. We dropped unused rules, such as the now-superfluous magic system. We simplified the combat to reduce the number of die rolls (a necessity for the Dungeon Master, who often had dozens of undead monsters in any combat). Our game was gradually becoming less Dungeons and Dragons and, step-by-step, turning into something new and unique. It was becoming ZOMBIEPOCALYPSE.