I was playing last night, and I am pretty sure I temporarily broke the game. I was playing in NG+, going to kill the Fool's Idol boss. I first killed her, and she disappeared, but did not evaporate into green mist, and did not get the message "YOU REVIVED…DEMON IS SLAIN…etc..". I could exit the fog, and re enter, which would oddly enough RESTART the Fool's Idol boss event. I killed her again and the same thing happened: nothing. I reloaded my profile, and the same thing happened again. I then tried to exit the fog, then reload profile, which did not work. What finally did work was quitting the game entirely back to the XMB, and starting the 3-1 at the beginning
What I think caused this bug
The first time I killed her, she may have been phasing out (how she turns yellow and phases to different spots around the room) and for some reason the game may have thought she was stuck in phasing??
This has only happened to me this one time, so I could not test why it happened.
I ended up dying a few times getting back to her in stupid frustration, when I should have gotten the kill on my first try. I made sure not to kill her while she was about to disappear after reloading the entire game, and it seemed to have fixed it. I haven't seen anything on this site about this happening to anyone else.