Whilst playing Hardcore Mode, I had the misfortune to die.
I clicked "Delete World" and was greeted with an "Are you sure you want to quit?" message. My choices were "Quit to Title" and "Respawn." Out of curiosity, I clicked "Respawn."
My game immediately froze and no actions could be made. Since I was in fullscreen, I couldn't force quit the application, and had to restart the computer to close the game.
I'm not sure if this is a OSX only problem, or in all versions, but it does need addressing.