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Yeah, not because it has Klutz, but because Thief can steal Mail normally (!). When I tested your scenario it worked, but Trick still failed, so I used a non-Klutz Pokemon and the same thing happened.
I'm guessing years ago I was trying to get a wild Pokemon to steal my Mail, but wild Pokemon can't steal any of the player's items at all in Gen 5. Whoops! Looks like it's only untrickable/unbestowable.
I'm guessing years ago I was trying to get a wild Pokemon to steal my Mail, but wild Pokemon can't steal any of the player's items at all in Gen 5. Whoops! Looks like it's only untrickable/unbestowable.