Hopper: java.util.ConcurrentModificationException

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at jq.c(SourceFile:203)
	at jr.b(SourceFile:110)

---- Minecraft Crash Report ----
// I'm sorry, Dave.

Time: 8/2/13 9:49 AM
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at jq.c(SourceFile:203)
	at jr.b(SourceFile:110)
	at net.minecraft.server.MinecraftServer.t(SourceFile:462)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at bkw.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hh.run(SourceFile:582)


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

-- Head --
Stacktrace:
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:793)
	at java.util.HashMap$KeyIterator.next(HashMap.java:828)
	at jq.c(SourceFile:203)
	at jr.b(SourceFile:110)

-- Affected level --
Details:
	Level name: Generated World
	All players: 0 total; []
	Chunk stats: ServerChunkCache: 994 Drop: 425
	Level seed: 141107
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (42,21,179), Chunk: (at 10,1,3 in 2,11; contains blocks 32,0,176 to 47,255,191), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 4097 game time, 4097 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 116650 (now: false), thunder time: 23289 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:462)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at bkw.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hh.run(SourceFile:582)

-- System Details --
Details:
	Minecraft Version: 1.6.2
	Operating System: Mac OS X (x86_64) version 10.8.4
	Java Version: 1.6.0_51, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 163561424 bytes (155 MB) / 241233920 bytes (230 MB) up to 4286251008 bytes (4087 MB)
	JVM Flags: 1 total; -Xmx4G
	AABB Pool Size: 1087 (60872 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 3, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 159 (8904 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Player Count: 0 / 8; []
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.