Having fun with Counter glitches (Generation I)
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This video combines multiple Counter glitches, including:
1) If the last attack used in memory was a Normal or Fighting type move, was not Counter and dealt damage (even if it was from another battle) and the opponent uses a healing item, Counter will deal twice as much damage as the last attack and it looks like you're countering a healing item. This does not work with Bruno's X Defenses (and probably other X items) as it is lower in priority than Counter.
2) You can do number 1 but the other way around (the enemy countering your items). You don't have to use a healing item, as it works with X items. If Counter is used in succession, then its damage will 'double' every turn.
Number 2 is also seen in this video. https://www.youtube.com/watch?v=_pyUJZtiGNU
3) If you meet the last attack conditions in 1) and switch Pokémon, the opponent's Counter will 'Counter your switch' and this can be repeated for more than one switch if the opponent keeps using Counter.
At least 1) and 2) were not fixed in Pokémon Yellow.
I'd like to thank Pum for bringing number 2 to my attention (their channel https://www.youtube.com/user/wowqueprecioss), and Háčky for his research. For more technical details about the Counter glitches, see this thread. http://forums.glitchcity.info/index.php/topic,6993.0.html