Hardcore Death Screen displayed on new world creation screen instead of normal GUI background (dirt). This can occur even after the world file has been manually deleted in the 'Select World' screen. (So no, I can't send you the world file it was deleted before the pic below.)

This shows that on world exit, Java is not properly clearing memory and may be related to some of the world gen corruption that can happen when creating then deleting many worlds in a single play session. (Just a thought.)

PS: This has actually been going on since at least 1.9 when spectator mode was added, just never though much about it. I'd like to note that the 'Delete World' button still only quits and does not (and never has) delete the world file.