So I was trying to design a way for Slow to make a fancy sch-mancy staircase for the new FTB map, and I thought "Hey! Lets try lines of blocks with stairs at the end of them coming from advanced drawbridges!" So I did that, The railcraft stairs seemed to work fine (other than, well, not existing and not rendering), but when I put vanilla brick stairs in there at the end, I looked at the spot it was supposed to be occupying and I crashed.
Dat Crashlog yo: http://pastebin.com/raasaJ68
Have a good day and happy modding!

Also happened to me.. crash log: http://pastebin.com/gK5Pu1Bh

Doesn't crash in the current sourcecode