Speedrunners are obsessed with finishing games in the fastest time possible, but they don't always play fair - any glitches in a game which allow the player to complete it in a faster time are considered by many to be perfectly legit, although they are usually classified differently to "normal" speed runs.
Even so, these glitch runs are often more interesting than standard ones as they show how a game can be exploited and even broken in certain ways to make it think it has been completed. Super Mario Land 2 is an excellent example of this - as renowned speedrunner Chris Grant shows in the video below.
By dropping through a level at the right point, Grant is able to trick the game into thinking he's beaten it - shaving plenty of time off the usual speedrun standard. What makes this particular glitch so interesting is that Mario falls into what appears to be a random sea of in-game assets and objects, including blocks, pipes, water and even text. Interacting with these elements triggers unexpected results, including a complete reboot of the game. The catch is that the exploit is apparently limited to very early versions of the game.
How people find these things, we'll never know.