Game crashed as I was traveling along an already generated chunk

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here

I was riding an armored horse to a desert outpost, mapping already generated and explored territory. The map I was using was linked to a map in a map room.

What actually happened was...:
Describe what happened here

The game crashed.

Steps to Reproduce:
Not really sure. I've noticed choppy behavior when forcing the game to generate new terrain (i.e. traveling to ungenerated chunks), but for the most part travel in terrain generated prior to upgrading to 1.8 has been smooth. This was choppy for some reason.

Now for the big weirdness! My world was deleted. Fortunately I saved it off somewhere.

1. Put a step by step guide on how to trigger the bug here
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