Randomly, the game will load a level, run the server internally, but doesn't close the main menu gui. This theoretically allows one two load two levels at once, but I have not tested this, as it probably would corrupt the worlds.

Please don't say you cannot reproduce this, as it is (from what I can see)
completely random, so it may take a very long time to reproduce.

Also, just before a level loads, the screen goes to the main menu, then to the game itself. I think this is what is causing the bug.