Hopper: java.lang.ClassCastException: mq cannot be cast to jj

java.lang.ClassCastException: mq cannot be cast to jj
	at jk.a(SourceFile:10)
	at ef.a(SourceFile:164)

---- Minecraft Crash Report ----
// Shall we play a game?

Time: 10/25/13 9:51 AM
Description: Ticking memory connection

java.lang.ClassCastException: mq cannot be cast to jj
	at jk.a(SourceFile:10)
	at ef.a(SourceFile:164)
	at ms.c(SourceFile:134)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at bsk.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at ky.run(SourceFile:617)


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

-- Head --
Stacktrace:
	at jk.a(SourceFile:10)
	at ef.a(SourceFile:164)

-- Ticking connection --
Details:
	Connection: ef@7abb0257
Stacktrace:
	at ms.c(SourceFile:134)
	at net.minecraft.server.MinecraftServer.u(SourceFile:529)
	at net.minecraft.server.MinecraftServer.t(SourceFile:441)
	at bsk.t(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:358)
	at ky.run(SourceFile:617)

-- System Details --
Details:
	Minecraft Version: 1.7.2
	Operating System: Mac OS X (x86_64) version 10.8.5
	Java Version: 1.6.0_51, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 55686696 bytes (53 MB) / 149696512 bytes (142 MB) up to 1069416448 bytes (1019 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 2623 (146888 bytes; 0 MB) allocated, 2463 (137928 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 346 (19376 bytes; 0 MB) allocated, 339 (18984 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.