After Updating to Buildcraft 6.0.12:
http://goo.gl/2XnYTA

Crash on startup:
http://pastebin.com/MwYYYL18

This seems to be a misuse on the API by one of your mods (the log says com.rwtema.extrautils). It's hard to see how we can protect the code from this - you'd have to talk to the mod author. We're happy to help if there's any question on http://mod-buildcraft.com/forums/index.php?board=5.0