The classic unintended mechanic is of course Bunnyhopping, which involves jumping and strafing in order to break the speed cap imposed on you in first person games like Quake and Half-Life 1.
In Half-Life 2, Valve sought to remove bunnyhopping from the game, and they did this by adding negative force to the player when they attempt to bunnyhop. However, there's no cap to the amount of negative speed you can get, meaning if you just turn around while bunnyhopping, you end up going even faster then regular bunnyhopping would've allowed.