1. Create a new world. Its game mode is "hardcore" (set it at first) and world type is "debug mode" (Hold shift and press "world type" button when it is "customized"). Now the game mode become "spectator mode" and you can't change.
2. Now go to the "select world" screen. There is a red "hardcore mode" under the world's name instead of "spectator mode". But it is still "spectator mode" if you load it.
3. Enter "/kill" to kill yourself. Then you can see "delete world" (but your gamemode is spectator mode), click it.
4. Click "respawn", then Minecraft will be stuck (not crash) in the "game over" screen.
5. If you go to the "select world" screen again, the world will disappear.