Well that explains what broke our server a few days ago when I was setting up an MFR tree farm with vacuum hoppers to pull dropped items off the ground. I thought I had messed something up with the miroblocks I was using (i had just put a vacuum hopper next to both a cover and the planter). Good thing I keep backups. I'll let my players know to avoid that one for now.
World crashing one this one.
Found it out the hardway with my singleplayer world, put a sewer down, put a vacuum hopper above it, the moment i went to tell the hopper to output to the sewer, insta-crash, everytime i tried to get back into the world, insta-crash.
It wasn't until i actually went into the OpenBlocks config and set the Vacuum Hopper ID to 0 that i was able to get back into my world, then i quit, set it back to what it should have been and all was right again.
Crash report from this incident is here: http://pastebin.com/iJmwphP6
Since that fateful day, i have avoided putting vacuum hoppers near MFR blocks.