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Phil Erwin's site has died with Geocities, and as such isn't available any more.
As for Missingno, for duping items, the glitch kicks in during the half-second pause just before the battle screen. That pause is actually the game doing a "quicksave", which is part of the corruption.
I've gone over the game-destroying possibilities of Missingno many times before - it is a real risk, albiet a miniscule one. I know of less than 15 reports on GameFAQs in the past of that sort of extreme corruption (one of them being my own). As such, there's always a disclaimer when I talk about Missingno.
How the Missingno duping glitch works.
When you encounter Missingno, the game doesn't quite know how to handle it, and as a result numerous sections in memory get modified. In the case of the memory location determining the amount of items the 6th Item slot has, the Most Significant Bit gets set to "1" - In laymans terms, if there's less than 127 of that particular item in the slot (which there will be unless you've just done the glitch), then it will add 127 to it.
Where logic won't work - I will
Pal Gamers unite - http://www.gamefaqs.com/boards/gentopic.php?board=277