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http://replay.pokemonshowdown.com/gen3ou-846308457
In ^that^ replay (turns 33-34 and turns 45-46), Block and Mean Look each stayed in effect after Snorlax and Jynx left the field.
I'm not certain, but I don't think that's how it's supposed to work...right? Lol
https://pokepast.es/88bae3df344b0fba
Substitute users are probably the biggest threats.
Phazers like Skarmory, Suicune or Swampert can also cause problems, so keep Magneton as healthy as possible.
Magneton doesn't have Thunder Wave or Toxic because they could interfere with Jynx's Lovely Kiss...