Pokemon R/B/Y - Substitute + Confusion Glitch
136Short version:
In Pokemon Red, Blue, and Yellow, if a Pokemon under a substitute hurts itself in its confusion, damage will be dealt to the other Pokemon's substitute instead.
Long version:
In Pokemon Red, Blue, and Yellow, there are two functions that handle the damage dealt to a non-substitute target; one applies damage to the player, and the other applies damage to the enemy. Normally, which function is called depends on whether the turn belongs to the player or to the enemy (if it's the player's turn, call the second function, and vice versa). But if the game needs to handle self-inflicted confusion damage, the opposite function is called.
However, there is only one function that handles the damage dealt to a substitute. This subroutine will be called by either of the two functions described above when the *target* has a substitute up. The 'whose turn' flag determines whether to deal damage to the player's or enemy's substitute. This overrides the fact that confusion damage should use the opposite logic in order to inflict damage to the attacker, leading to this interesting bug.
Notice how the *target* is the attacker if we are applying confusion damage, and the defender otherwise. This makes this bug even more intriguing because the subroutine will be called when the confused Pokemon has a substitute up, and thus damage will be "dealt" to the other Pokemon's substitute regardless of whether that Pokemon is actually under a substitute.