Edward Freedman

Apologies if this is duplicate information from an existing comment.

Here's the actual problem: When migrating an existing map from 1.7 to 1.8, not all of the existing portals stay lit... or if they do, they don't stay fully lit. (By lit I mean active; I wish I'd taken screenshots of portals with only one or two squares active.) Thus, when you step from the Nether to what you think is an existing, active portal, the game will find the obsidian from the previous portal, now deactivated, which blocks the spawn of a new active portal. As a result, a new active portal is spawned, sometimes right near the previous portal, sometimes further away.

Again, apologies if this comment duplicates a previous comment.

Good heavens, I've been playing this off and on for a year and I never noticed it was using the x86 Java. I'm installing the x64 Java and will change my MC launcher to use it (Windows itself is already x64, inspite of what the environment states). Thank you for spotting that obvious blunder!

Crash report.