Ok, I stumbled upon this bug accidently. What happened was I was in the Nether Portal animation, and I got logged out forcefully. Upon logging back, I came to find that my world isn't functioning right. I did more testing and it isn't 100% working. (No pictures because, well, duh)

Steps to Reproduce:
1. Enter the Nether Portal from the Nether side
2. Wait until you get into the loading terrain screen
3. Close Minecraft entirely.
4. Launch Minecraft
5. Try launching your world

Expected Result: You log into the last spot you saved at.
Observed Result: You couldn't access your world.

(Keep in mind, this won't work 100% of the time, so keep trying until you get a message saying that Minecraft has issues loading into the world, and please make a copy of your favorite worlds when reproducing this bug)