When using MystCraft books to teleport between worlds I've occasionally been crashing with this error.
Display Spoiler
2012-10-30 07:04:18 [INFO] [STDERR] 2012-10-30 07:01:29 [INFO] [STDOUT] Loading World: local/New Turtilia
2012-10-30 07:04:15 [INFO] [ForgeModLoader] Loading dimension 17 (New Turtilia) (axv@77324fc2)
2012-10-30 07:04:15 [INFO] [STDOUT] Unable to find spawn biome
2012-10-30 07:04:15 [INFO] [STDOUT] Unable to find spawn biome
2012-10-30 07:04:18 [INFO] [STDOUT] Sending serverside check to: bbqr0ast
2012-10-30 07:04:18 [INFO] [STDOUT] Loading World: local/New Turtilia
2012-10-30 07:04:18 [INFO] [STDERR] java.util.ConcurrentModificationException
2012-10-30 07:04:18 [INFO] [STDERR] at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
2012-10-30 07:04:18 [INFO] [STDERR] at java.util.ArrayList$Itr.next(Unknown Source)
2012-10-30 07:04:18 [INFO] [STDERR] at up.p(World.java:2590)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.api.Direction.applyToTileEntity(Direction.java:61)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.common.EnergyNet.getValidReceivers(EnergyNet.java:536)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.common.EnergyNet.discover(EnergyNet.java:425)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.common.EnergyNet.emitEnergyFrom(EnergyNet.java:213)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.common.TileEntityBaseGenerator.sendEnergy(TileEntityBaseGenerator.java:220)
2012-10-30 07:04:18 [INFO] [STDERR] at ic2.common.TileEntityBaseGenerator.g(TileEntityBaseGenerator.java:131)
2012-10-30 07:04:18 [INFO] [STDERR] at up.h(World.java:1954)
2012-10-30 07:04:18 [INFO] [STDERR] at gr.h(WorldServer.java:452)
2012-10-30 07:04:18 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.q(MinecraftServer.java:634)
2012-10-30 07:04:18 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.p(MinecraftServer.java:565)
2012-10-30 07:04:18 [INFO] [STDERR] at axv.p(IntegratedServer.java:110)
2012-10-30 07:04:18 [INFO] [STDERR] at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:471)
2012-10-30 07:04:18 [INFO] [STDERR] at ep.run(SourceFile:539)
My random guess is that IC2 isn't realizing I'm on a new world and is trying to continue EnergyNet calculations it started on the previous world. The bug isn't fatal, and the world I teleported to loads fine once I arrive.
I extracted this from the forge log file (as this bug only happens occasionally) and I'll post the full error if I can replicate it again.
