I was fighting siren for the mission, after getting her down to about 1/4 hp she simply vanished, this was very disappointing as I had spent about 5-10 minutes fighting her already, tried to click on the ??? After it happened, and the game gave me a wait message, waited a bit longer and clicked again, and the fight started all over (at which point I just warped and exited the game, SO flustered)