A few months ago I was just travelling across the ocean on a boat in my survival world when a bunch of blue shapes started freaking out all over the window. The game crashed and when I reloaded it my world was missing from the World's List. I checked the saves folder and the world was still there, but it won't appear in the World's List in-game.
I sent the world to my sister's PC with identical specs and it wouldn't show up for her either, but she has never experienced the blue shapes before.
World download is here, it's too big for attachment.