Crash Log.

It should be noted that I was unable to reproduce this. Only the first attempt at generating a world failed, the second seemingly worked.

My friend was having issues too however he wasn't crashing. He said it was just locking up.

I realize this happened during terrain generation, but it is unrelated to ExtrabiomesXL.

Those were the only two mods installed, so you're saying it's Buildcraft? Alright.

Actually, the logs show that the crash happened in unmodded vanilla code. I am making no conclusions.