Crash when a certain player joins / Exception ticking world: ConcurrentModificationException

(Made sure the server was pure vanilla this time. Reproduced crash.)

---- Minecraft Crash Report ----
// I feel sad now :(

Time: 3/18/13 9:26 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextEntry(Unknown Source)
	at java.util.HashMap$KeyIterator.next(Unknown Source)
	at iy.b(SourceFile:196)
	at iz.b(SourceFile:122)
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)


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 iy.b(SourceFile:196)
	at iz.b(SourceFile:122)

-- Affected level --
Details:
	Level name: world
	All players: 6 total
	Chunk stats: ServerChunkCache: 6138 Drop: 146
	Level seed: 4709343022063829704
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (108,64,252), Chunk: (at 12,4,12 in 6,15; contains blocks 96,0,240 to 111,255,255), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 204182393 game time, 195675149 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 30350 (now: false), thunder time: 87283 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.r(SourceFile:454)
	at hz.r(SourceFile:255)
	at net.minecraft.server.MinecraftServer.q(SourceFile:397)
	at net.minecraft.server.MinecraftServer.run(SourceFile:331)
	at gp.run(SourceFile:573)

-- System Details --
Details:
	Minecraft Version: 1.5
	Operating System: Linux (amd64) version 3.4.24-nfo
	Java Version: 1.7.0_11, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 62746016 bytes (59 MB) / 550895616 bytes (525 MB) up to 1908932608 bytes (1820 MB)
	JVM Flags: 1 total; -Xmx2048M
	AABB Pool Size: 16910 (946960 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: 20344 (1139264 bytes; 1 MB) allocated, 0 (0 bytes; 0 MB) used
	Player Count: 6 / 50
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)