I was playing in a hardcore world and after dying to a creeper, the "Spectate World" and "Delete World" options popped up. I clicked "Delete World". It returned me to the main menu where the rotating background screen had disappeared and turned into a whitish, gray screen behind the menu buttons. Opening the singleplayer game button and returning to the menu didn't clear the issue. The hardcore world still appeared in the worlds screen and I got curious and attempted to open it. The game then froze with a dark gray screen and had to be killed in the Task Manager.

How to replicate:
Create a hardcore world, die and click the "delete world" option.