During a survival playthrough, I entered a Nether portal, which I had already used previously, going from the overworld to the Nether. I was instantly killed when I was supposed to be spawned (not suffocation - death was instant despite my health being full), and the screen under the "you died" overlay was completely black, indicating that the area hadn't loaded in correctly. When I selected the option to respawn, it brought up the "generating terrain..." loading bar, and then froze. 

I closed the game out and reloaded it. Because I hadn't set a spawn, I was back at the spawn of the world - exactly what should happen, but I figured I would specify. 

Finally, I went into Creative mode to investigate. After entering the Nether from a different portal and locating the one that had killed me, I found that it was not lit from the Nether side. My items were inside the portal frame. There was no evidence of an explosion, or anything that would've killed me. Nothing of this nature on the overworld side, either. This, along with the fact that my screen was black when I died rather than showing the Nether, convinced me that it was some kind of loading error.