Lava retrieval systems using Mystcraft portals and railcraft tankcarts are popular, and I use such a one for retrieving oil and lava. I am currently using v4 of the Direwolf 20 mod pack for FTB. The crash occurs occasionally when a railcraft tank-cart comes through a portal and appears at its spot. Compatibility issue?
Sample crash report: http://pastebin.com/Dhds6cpZ

No, its not a compatibility issue. Its just Mystcraft derping up somehow.

Its some kind of concurrent modification error from multiple threads changing the same list.

Until someone can prove that its has anything to do with me, I'm just ignoring it.