at cpw.mods.fml.common.network.internal.FMLProxyPacket.func_148833_a(FMLProxyPacket.java:101)
Ahh you hit one of the classic MC/Forge bugs. Sadly there is nothing that can be done about this short of editing Forge itself (this is code that loads ‘before’ even asm mods). When you hit that, just keep trying to reload until it works. Some computer setups and mod combinations make it more likely but inherently it is so random that it is near undiagnoseable. In general though the more mods there are then the more likely it is to occur as it is a timing issue between the client and server parts of the network code when both are running in the client itself. A work-around if it happens often is to run a dedicated server and connect the client to it.