Re: Missing Number
As Marco said, Missing Number (More commonly reffered to as "MissingNo.") is a game glitch of RBY. Some believe it was the original test Pokemon, along with its other variations, but obviously their "test sprite" was removed, showing a glitched block instead.
Other than glitch your game it could also be manipulated to duplicate Items, giving you infinite quantities. MissingNo has not been present in any game since RBY without use of a cheat device, and even then it cannot duplicate items.
This has been discussed quite a lot in the game boards.
As long as we continue to fight, and destroy, we are forever prisoners of our ignorance and foolishness; true 'freedom' does not exist