I was testing out the 1.8.2 pre 1 snapshot and created a new world.

I built a nether portal in creative mode and traveled through it.

I flew around for a while and then placed another nether portal to go back to the overworld.

When I returned i could not move out of the nether field. I could punch blocks and open the creative inventory and place blocks, but the game wouldn't let me move away.

In addition my screen was rapidly moving up and down much faster than jumping but I wasn't pressing space

I took a look at the F3 debug screen and my Y coordinate was rapidly changing by little increments up and then back down and up again.

So I quit the game and reloaded the world. My character is still stuck in the nether field. and the Y coordinate looks funny. It is stuck at 39.96573.

I can move from 1250.300 to 1250.700 in the x coordinate and -16.300 to -16.700 in the z coordinate, but I cannot leave block 1250 39, -17.

After I use the /kill command I can move freely again.

After I respawned I teleported to a brand new area of the map and build another portal to the nether, flew around and built another portal to the overworld and went through just fine.

I don't know if I can reproduce this...