Write in the chat and it lagg and crash.

---- Minecraft Crash Report ----
// You're mean.

Time: 3/6/2556, 8:08 น.
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
	at java.util.HashMap$KeyIterator.next(Unknown Source)
	at abn.a(SourceFile:70)
	at je.b(SourceFile:121)
	at net.minecraft.server.MinecraftServer.s(SourceFile:465)
	at net.minecraft.server.MinecraftServer.r(SourceFile:408)
	at bih.r(SourceFile:125)
	at net.minecraft.server.MinecraftServer.run(SourceFile:342)
	at gu.run(SourceFile:585)


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

-- Head --
Stacktrace:
	at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
	at java.util.HashMap$KeyIterator.next(Unknown Source)
	at abn.a(SourceFile:70)
	at je.b(SourceFile:121)

-- Affected level --
Details:
	Level name: Copy of New World
	All players: 0 total; []
	Chunk stats: ServerChunkCache: 423 Drop: 0
	Level seed: -268061311838734820
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-44,64,256), Chunk: (at 4,4,0 in -3,16; contains blocks -48,0,256 to -33,255,271), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 24 game time, 24 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 31583 (now: false), thunder time: 139863 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.s(SourceFile:465)
	at net.minecraft.server.MinecraftServer.r(SourceFile:408)
	at bih.r(SourceFile:125)
	at net.minecraft.server.MinecraftServer.run(SourceFile:342)
	at gu.run(SourceFile:585)

-- System Details --
Details:
	Minecraft Version: 13w22a
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.7.0_21, Oracle Corporation
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
	Memory: 377027752 bytes (359 MB) / 905293824 bytes (863 MB) up to 1037959168 bytes (989 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 5390 (301840 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: 524 (29344 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.