I lit a Nether portal in my world and went straight through. I spent about a minute in there. When I came out I discovered that, about 30 blocks from the portal, there were random 'pieces' of the Nether that had replaced the Overworld blocks. There was even floating lava and quartz in the sky.

When I tried to enter that little Nether section, I couldn't. It made my character sort of 'spaz out.'

I just relogged and, yep, it's back to normal.... lol