What I expected to happen was...:

I was digging and flooring a new nether tunnel to the Realm North-South tunnel at Y = 115. 

I expected to hear the usual sounds of the Nether. I also expected to see zombie pig men spawn spontaneously in the tunnel.

What actually happened was...:
While I was away from keyboard for a few minutes, getting a drink of water, I was killed by a Ghast fireball through the intact wall of the nether tunnel. There was a game message on the screen reporting the manner of my death.

Steps to Reproduce:
I've not attempted to reproduce the bug.