Hopper: java.lang.ClassCastException: <unknown> cannot be cast to acm

java.lang.ClassCastException: <unknown> cannot be cast to acm
	at acm.equals(SourceFile:25)
	at java.util.HashMap.put(Unknown Source)
	at adf.a(SourceFile:49)
	at kd.b(SourceFile:108)

---- Minecraft Crash Report ----
// Everything's going to plan. No, really, that was supposed to happen.

Time: 9/7/13 5:51 AM
Description: Exception ticking world

java.lang.ClassCastException: <unknown> cannot be cast to acm
	at acm.equals(SourceFile:25)
	at java.util.HashMap.put(Unknown Source)
	at adf.a(SourceFile:49)
	at kd.b(SourceFile:108)
	at net.minecraft.server.MinecraftServer.t(SourceFile:455)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bng.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hs.run(SourceFile:575)


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

-- Head --
Stacktrace:
	at acm.equals(SourceFile:25)
	at java.util.HashMap.put(Unknown Source)
	at adf.a(SourceFile:49)
	at kd.b(SourceFile:108)

-- Affected level --
Details:
	Level name: 1.7 Epic World
	All players: 1 total; [kg['rpattykerbs'/25383, l='1.7 Epic World', x=-175.35, y=77.00, z=-184.12]]
	Chunk stats: ServerChunkCache: 734 Drop: 0
	Level seed: 106851417
	Level generator: ID 03 - amplified, ver 0. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-16,64,76), Chunk: (at 0,4,12 in -1,4; contains blocks -16,0,64 to -1,255,79), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 43903 game time, 43903 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 66367 (now: false), thunder time: 127303 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.t(SourceFile:455)
	at net.minecraft.server.MinecraftServer.s(SourceFile:398)
	at bng.s(SourceFile:124)
	at net.minecraft.server.MinecraftServer.run(SourceFile:332)
	at hs.run(SourceFile:575)

-- System Details --
Details:
	Minecraft Version: 13w36a
	Operating System: Windows 8 (amd64) version 6.2
	Java Version: 1.7.0_25, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 122176048 bytes (116 MB) / 277086208 bytes (264 MB) up to 954466304 bytes (910 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G
	AABB Pool Size: 2808 (157248 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Suspicious classes: [com.ibm.icu.text.ArabicShapingException], [gnu.trove.TIntCollection], [gnu.trove.impl.HashFunctions, PrimeFinder], [gnu.trove.impl.hash.THash, TPrimitiveHash, TIntIntHash], [gnu.trove.iterator.TIterator, TAdvancingIterator, TIntIntIterator], [gnu.trove.map.TIntIntMap], [gnu.trove.map.hash.TIntIntHashMap], [gnu.trove.procedure.TIntIntProcedure], [gnu.trove.set.TIntSet]
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 5358 (300048 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Player Count: 1 / 8; [kg['rpattykerbs'/25383, l='1.7 Epic World', x=-175.35, y=77.00, z=-184.12]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.