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Marksman MAY have been talking about Missingno...if you were, <shocked and horified gasp>
Have you never played the original (or original ENGLISH) versions of the game? Ah yes, Missingno...good times...:P
Basically just a glitch Pokemon that allowed you to get Pokemon above level 100 or clone items such as Master Balls and Rare Candy's.
I actually only realised how to pronounce Missingno last year...I had a friend in school who had Pokemon Blue but DIDN'T KNOW ABOUT MISSINGNO (and used it to bamboozle his equally ignorant cousins) and told him about it, and realised I had been pronouncing it as Missingo.