[MC-21833] game crash

Put the summary of the bug you're having here

What I expected to happen was...:
What actually happened was...:
The game will no longer load. It immediately crashes and generates the error when trying to load the game.

Steps to Reproduce:
The game crashes as soon as I try to launch it.
Has been ongoing for a very long time and I believe there is a corruption in the map. Initially the game would load but would crash, often when attempting to travel to the "western" part of my world. I tended to avoid venturing off in that direction and things seemed to work ok for a time. Now I cannot even load my world.

---- Minecraft Crash Report ----
// My bad.

Time: 03/07/13 10:27 PM
Description: Exception in server tick loop

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
	at java.util.HashMap$ValueIterator.next(Unknown Source)
	at aii.a(SourceFile:73)
	at aeo.a(SourceFile:425)
	at jo.a(SourceFile:168)
	at adm.a(SourceFile:732)
	at jo.c(SourceFile:105)
	at jn.<init>(SourceFile:29)
	at jm.a(SourceFile:208)
	at jm.a(SourceFile:240)
	at hk.a(SourceFile:138)
	at hk.c(SourceFile:185)
	at hk.a(SourceFile:82)
	at bkr.b(SourceFile:73)
	at net.minecraft.server.MinecraftServer.t(SourceFile:488)
	at net.minecraft.server.MinecraftServer.s(SourceFile:405)
	at bko.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:339)
	at hf.run(SourceFile:582)


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

-- System Details --
Details:
	Minecraft Version: 1.6.1
	Operating System: Windows 7 (amd64) version 6.1
	Java Version: 1.7.0_15, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 196291984 bytes (187 MB) / 460193792 bytes (438 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 308 (17248 bytes; 0 MB) allocated, 308 (17248 bytes; 0 MB) used
	Suspicious classes: No suspicious classes found.
	IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 36 (2016 bytes; 0 MB) allocated, 36 (2016 bytes; 0 MB) used
	Player Count: 1 / 8; [js['kwyjibo'/19, l='World1', x=-531.55, y=14.00, z=-321.76]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.