Hopper: java.util.ConcurrentModificationException

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-1751785/

Crash:

java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
	at java.util.ArrayList$Itr.next(Unknown Source)
	at qh.a(SourceFile:400)
	at sd.a(SourceFile:812)
	at cwk.x(SourceFile:142)
	at net.minecraft.server.MinecraftServer.run(SourceFile:430)
	at java.lang.Thread.run(Unknown Source)

The following is the first clean report made public about this crash:

---- Minecraft Crash Report ----
// But it works on my machine.

Time: 23/07/14 17:19
Description: Exception in server tick loop

java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
	at java.util.ArrayList$Itr.next(Unknown Source)
	at qh.a(SourceFile:400)
	at sd.a(SourceFile:812)
	at cwk.x(SourceFile:142)
	at net.minecraft.server.MinecraftServer.run(SourceFile:430)
	at java.lang.Thread.run(Unknown Source)


A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

-- System Details --
Details:
	Minecraft Version: 14w30b
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.7.0_60, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 987443520 bytes (941 MB) / 1514668032 bytes (1444 MB) up to 3817865216 bytes (3641 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx4G
	IntCache: cache: 1, tcache: 1, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Player Count: 0 / 8; []
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-1751785/69650527/