Minecraft Closes

I just opened Minecraft, clicked my world, and after a second of loading, it closed. I tried again and again and it keeps automatically closing. And gave me this message on the Minecraft Launcher:

---- Minecraft Crash Report ----
// I'm sorry, Dave.

Time: 7/6/13 3:17 PM
Description: Exception in server tick loop

java.lang.ClassCastException: aqn cannot be cast to amr
	at ams.<init>(SourceFile:33)
	at amr.a(SourceFile:372)
	at amr.a(SourceFile:334)
	at adm.a(SourceFile:395)
	at abr.f(SourceFile:307)
	at aim.a(SourceFile:384)
	at aim.a(SourceFile:513)
	at agd.a(SourceFile:392)
	at aio.a(SourceFile:29)
	at aii.a(SourceFile:76)
	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: Mac OS X (x86_64) version 10.6.8
	Java Version: 1.6.0_31, Apple Inc.
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
	Memory: 90892496 bytes (86 MB) / 217636864 bytes (207 MB) up to 1069416448 bytes (1019 MB)
	JVM Flags: 1 total; -Xmx1G
	AABB Pool Size: 3231 (180936 bytes; 0 MB) allocated, 3231 (180936 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: 346 (19376 bytes; 0 MB) allocated, 346 (19376 bytes; 0 MB) used
	Player Count: 1 / 8; [js['RainCake'/298, l='Rain's Oasis-', x=-795.75, y=67.00, z=431.89]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

Additional Info: I am using a Mac.