Got this error when I reloaded a world:
java.lang.NullPointerException
at ic2.common.TileEntityCable.onCreated(TileEntityCable.java:90)
at ic2.common.TileEntityBlock$1.tickCallback(TileEntityBlock.java:27)
at mod_IC2.OnTickInGame(mod_IC2.java:1377)
at ic2.platform.Ic2.onTickInGame(Ic2.java:347)
at ModLoader.onTick(ModLoader.java:1102)
at EntityRendererProxy.b(EntityRendererProxy.java:22)
at net.minecraft.client.Minecraft.x(Minecraft.java:869)
at net.minecraft.client.Minecraft.run(Minecraft.java:747)
at java.lang.Thread.run(Unknown Source)
However, when I reloaded again, I got no errors.
I took a look and it seems that sometimes you get unlucky and it calls onCreated before it is valid, and its worldObj is still null (I think).