The crash happens consistently for me when leaving the area when Mekanism fluid pipes are connected to two Forestry farms. Issue seems similar to #1204. It happens on a personal client in creative mode and on a survival server with the same mods list, save for the clientside only mods.

I have not had this error when only one farm is connected.
I have not had issues with Mekanism fluid pipes connected to other Mekanism devices.
I have not had this issue with the two IC2 machines and two mekanism devices connected to a fluid pipe, just so far when two forestry farms are connected, in case any of this helps.

Happens with both 6.0.4.42 and with 6.0.5.46

error log: http://pastebin.com/DM5c9nYr

ic2 is out of date. http://ic2api.player.to:8080/job/IC2_experimental/397/ is latest for 1.6.4

Thank you for pointing that out, I imagine it isn't the only outdated mod in this group. I just updated to ic2 397, the issue still occurs. Even the old IC2 seemed to be fine with Mekanism fluid pipes but it was worth trying since the second farm was a rubber farm. I think this is a Forestry/Mekanism issue. I posted here instead of Forestry since the exception seems to be coming from Mekanism. What I really should do is try to figure out which mods are necessary to test this and remove everything else. That may take some time, I'm going to work on trying that now.

Fixed for v7.