Surfing
This owes much to coders balancing out the game, and a freak streak of luck. In Tribes, the PC multiplayer shooter, a beta player discovered a technique that came to be known as 'skiing' - where (in basic terms) a bit of code dictating friction on slopes, when combined with repeated jumping, created enough momentum to power across the game's huge maps in a matter of seconds. Whole maps were dedicated to the trick, making for some ludicrous designs.
Surfing turned up in Counter Strike, another friction/slope combination that created an entirely new game mode among faithful players, complete with purpose-built maps that more closely resemble bonkers rollercoasters. Thankfully, the entertaining skiing trick stayed untouched for the entire Tribes series, and surfing also returned unmolested in Valve's revamped Counter Strike: Source. Are you reading, Blizzard? Sometimes these gameplay oddities are quirky bonuses, not bugs. Though fans of Enemy Territory: Quake Wars don't seem to agree.