I was doing a mission with my alt (yes i have a few of those) and one level was completely empty! every room except when everything came back to normal on a party button was gone. Wisp were invisible though. I was getting shocked by invisible wisps but not attacked by invisible monsters. I was 20% laggy and i havent encountered it anymore. i thought ill just put it out there.
note: i will add screenie when i get on desktop.