According to Master ZED's Bug FAQ, there's a bug that occurs when Gau tries to enter Rage but is turned into an Imp beforehand. Rage is a command that is not compatible with Imp, so Gau will Fight instead, and will not gain any of the statuses or immunities attached to the Rage; however, he will still become autonomous.
Normally, during Rage, on every attack the character makes, the properties of the monster in question are reloaded for the character. However, if that character has become an Imp, the property reload routine will ignore the fact that the character is still in a Rage and not even call Rage's reload routine. They will still attack anyway though, making this a dangerous bug. It's also the reason why Critical Hits if Imp doesn't work for characters. Once Imp status is healed, the Rager must still take a turn before the Raged monster's properties are restored to them because the Rage command isn't used during Imp (it's swapped for Fight).
I tried a fix that makes Rage compatible with Imp, thus giving Gau the elemental properties of the Rage as usual, while still ensuring he only attacks and doesn't try to cast any spells (they will fail because of the Imp status). However, there's now an issue if Gau Rages as a monster with immunity to the Imp status because the immunity will prevent the status from being added or removed, meaning Gau will remain an Imp until the battle ends or he is KO'd.
With this in mind, there are three possible ways to fix this. One is to make Rage ignore immunity to the Imp status, but this is probably not a good idea as it veers away from the developers' intentions. Another is to make Rage remove Imp status if immunity is included, but again, probably not a good idea as there may be situations where being an Imp is beneficial (e.g. Imp equipment). The third way, and the best IMO, is to make sure Gau does not become autonomous if he is an Imp.
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