As outlined over here, this issue affects both SSP and SMP. A crash is produced when hooking up forestry producers directly to Mekanism machines. In the example I used, a clockwork engine from Forestry next to a Basic Crusher caused this crash.

http://pastebin.com/QeCwSWyY

Fixed in 9.2.1.295

Mezz pushed a temporary fix to Forestry which stops the crash, however the log is now spammed every tick with the error.