The weirdest thing happened to me today in single player.
I finished building my nether portal and traveled through it. After harvesting some netherrack and glowstone, I stepped through the portal that I exited from in the Nether and was ported into the overworld of ANOTHER GAME THAT I PREVIOUSLY DELETED. Thinking that I couldn't get back to my current game, I deleted the save. In retrospect, I should have tried killing myself and see if I would reset back to my bed, but I totally neglected this possibility at the time.
Has this happened to anyone else?