I die when going through a portal

I built a nether portal, went through, got attacked, died and respawned. Fine. Then, when I tried to go back through the portal, I die before it's even finished loading by something I can't see (a magma cube, usually). I got upset, but respawned and tried again, this time I tried to attack while it was loading. I died anyway, then when I tried to respawn I got "The software was closed because an error occurred."
This is really annoying, that I can die in a loading screen. Why is this, and how can I fix it?