Hopper: java.util.ConcurrentModificationException

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:926)
	at java.util.HashMap$KeyIterator.next(HashMap.java:960)
	at jr.c(SourceFile:203)
	at js.b(SourceFile:110)

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 10/15/13 4:29 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:926)
	at java.util.HashMap$KeyIterator.next(HashMap.java:960)
	at jr.c(SourceFile:203)
	at js.b(SourceFile:110)
	at net.minecraft.server.MinecraftServer.t(SourceFile:463)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)


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

-- Head --
Stacktrace:
	at java.util.HashMap$HashIterator.nextEntry(HashMap.java:926)
	at java.util.HashMap$KeyIterator.next(HashMap.java:960)
	at jr.c(SourceFile:203)
	at js.b(SourceFile:110)

-- Affected level --
Details:
	Level name: kristinas new worla
	All players: 0 total; []
	Chunk stats: ServerChunkCache: 751 Drop: 265
	Level seed: -1198446518172102231
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-8,64,244), Chunk: (at 8,4,4 in -1,15; contains blocks -16,0,240 to -1,255,255), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 13048 game time, 13048 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 150140 (now: false), thunder time: 85097 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:463)
	at net.minecraft.server.MinecraftServer.s(SourceFile:406)
	at bkz.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:340)
	at hi.run(SourceFile:583)

-- System Details --
Details:
	Minecraft Version: 1.6.4
	Operating System: Mac OS X (x86_64) version 10.8.4
	Java Version: 1.7.0_40, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 81255136 bytes (77 MB) / 229113856 bytes (218 MB) up to 954728448 bytes (910 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 4706 (263536 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: 1169 (65464 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.