I've lost experience points to this, and it seems my brother quit a world on the PS3 a while back because of this too. I had built a portal, apparently there was a lava flow by the portal, which stopped a block below the portal, and you know what? I was taking damage before it finished loading me into the Nether! I tried going back several times, and it kept happening! I wasn't even touching the lava! It isn't fun.