Defense Up 1 Glitch - Page 1
So I just recently attempted to do a Weedle only run in Blue version when I came across this glitch which makes the run impossible. I call it the Defense Up 1 glitch, because all moves that have this effect triggers this. It also seems to effect Poison type moves only, I'm currently not sure if it effects other types. I attempted to beat Brock with my level 17 Weedle when I could no longer hit Geodude after it used Defense Curl within two turns. Poison Sting's accuracy rate is 100%, but the Defense_Up1 effect seems to decrease any Poison type move's accuracy, making each move miss. Is this oversight well known? I know I haven't been keeping up with this place lately. :P I also haven't checked if the defense_up2 causes this to happen, so it could very well be the same case.
It's an artifact of the damage formula, which can return 0 only in the case of quad-resistant moves (such as poison vs ground/rock). In Gen 1, the game says the attack missed on such cases. The defense boosts presumably pushed Geodude's defense high enough to trigger it. It is mentioned on this bulbapedia page, but I'm not sure I'd classify it as well known.
Ah I was looking at the wrong part of the main battle code, I figured that since it wasn't documented here yet that it wasn't too well known. Looks like I'll have another page to add to the wiki, given if it hasn't been added yet. :)