I was playing with my modpack on a server to test out things
I use SGcraft aka Stargates as the 'Main' form of movement in the Universe. Well out of pure stupidity I said...I'll go 'Back' through a stargate to see what would happen partly for grins and giggles and both client and server crashed:
Client implies funky and server implies SGcraft I guess with Mystcraft interaction perhaps
Client - http://pastebin.com/dFAt1sx6

I posted to SGcrafts designer about the server. Maybe its just Mystcraft interaction there...don't actually know but hope the client thing is relevant to you XD