Just met a perma invisible NIN in Frontline while attacking

[Nalien]

Thank you a lot for the answer, I hope no one found a way to exploit it.

As far as I know, the glitch is caused by the player experiencing it, so it would be hard for a player to really exploit it, since the person benefiting from it isn't the person initiating it. From what I've heard, it's caused when you attack a Ninja right as they gain their hidden effect; Client side, you hit them and cancelled Hide. Server side, the Ninja got Hide off and you shouldn't have been able to even target them. Client updates with server side information (namely, that the Ninja is hidden) and... Gets confused, I guess? Who knows with SEs spaghetti coding, either way the result is the Ninja remaining hidden for your client.