Setup: Direwolf20 pack 1.6.4, modified with newer versions of the mods and a few small additions.
Version: Hats 2.1.1, iChunUtil 2.4.0
Crashlog: http://codepad.org/BvpRSBFx
Code line: https://github.com/iChun/Hats/blob/master/hats/common/core/TickHandlerServer.java#L127
Exception: java.util.ConcurrentModificationException

I can't say for sure what could be causing such an exception, since I'm new to java and even more so, forge.
But maybe that remove in line 130 is resizing the map, causing it to fall out of sync within the loop?

Yep same thing. Hope this can be fixed soonish.

Same here, FTB - Direwolf20 1.6.4 pack, v1.0.14 with no other modifications. I was in the Overworld everytime it happened.

Description: Exception in server tick loop

java.util.ConcurrentModificationException
    at java.util.WeakHashMap$HashIterator.nextEntry(Unknown Source)
    at java.util.WeakHashMap$EntryIterator.next(Unknown Source)
    at java.util.WeakHashMap$EntryIterator.next(Unknown Source)
    at hats.common.core.TickHandlerServer.serverTick(TickHandlerServer.java:127)
    at hats.common.core.TickHandlerServer.tickEnd(TickHandlerServer.java:56)
    at cpw.mods.fml.common.SingleIntervalHandler.tickEnd(SingleIntervalHandler.java:34)
    at cpw.mods.fml.common.FMLCommonHandler.tickEnd(FMLCommonHandler.java:141)
    at cpw.mods.fml.common.FMLCommonHandler.onPostServerTick(FMLCommonHandler.java:274)
    at net.minecraft.server.MinecraftServer.func_71217_p(MinecraftServer.java:622)
    at net.minecraft.server.integrated.IntegratedServer.func_71217_p(IntegratedServer.java:129)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:484)
    at net.minecraft.server.ThreadMinecraftServer.run(SourceFile:583)

Unfortunately this ruined my save also somehow (after about the 4th crash, I guess I was lucky that first few times) - http://pastebin.com/F3VuXhjK.

Indigo Starfish - deleting the .dat in the save folder should fix it. (Just make a backup and remove hats temporarily until a fix is released.)

Also getting this crash, it's very annoying: http://pastebin.com/wMXKsfva

I'm getting the same crash.

IIRC different tick types are run concurrently. If that's the case, then TickHandlerServer.java:116 (worldtick) may be excecuted while iterating through mobHats (TickHandlerServer.java:127), causing a concurrent modification exception. playerKilledEntity also modifies mobHats, so you'd want to solve the concurrency issue there too, or I'm guessing it might crash some times when killing entities too.

3bca624

Also getting this error; all latest downloads.

http://pastebin.com/bm8V4KWb