I was playing Minecraft and it crashed. Below is the content of the crash file:
---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 1/9/13 9:05 PM
Description: Exception ticking world

java.lang.IllegalStateException: Entity is already tracked!
at ii.a(SourceFile:69)
at ii.a(SourceFile:50)
at ij.a(SourceFile:24)
at yc.a(SourceFile:842)
at in.a(SourceFile:580)
at yc.d(SourceFile:834)
at yp.a(SourceFile:138)
at in.b(SourceFile:113)
at net.minecraft.server.MinecraftServer.r(SourceFile:736)
at net.minecraft.server.MinecraftServer.q(SourceFile:679)
at bdz.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:613)
at fy.run(SourceFile:849)

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

– Head –
Stacktrace:
at ii.a(SourceFile:69)
at ii.a(SourceFile:50)
at ij.a(SourceFile:24)
at yc.a(SourceFile:842)
at in.a(SourceFile:580)
at yc.d(SourceFile:834)
at yp.a(SourceFile:138)
at in.b(SourceFile:113)

– Affected level –
Details:
Level name: New World
All players: 1 total; [iq['allam326'/224, l='New World', x=297.32, y=176.00, z=643.13]]
Chunk stats: ServerChunkCache: 1066 Drop: 0
Level seed: -4963525807504175116
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (212,64,252), Chunk: (at 4,4,12 in 13,15; contains blocks 208,0,240 to 223,255,255), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 956380 game time, 956380 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 93273 (now: false), thunder time: 105437 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.r(SourceFile:736)
at net.minecraft.server.MinecraftServer.q(SourceFile:679)
at bdz.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:613)
at fy.run(SourceFile:849)

– System Details –
Details:
Minecraft Version: 1.4.7
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.6.0_35, Sun Microsystems Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Sun Microsystems Inc.
Memory: 162331464 bytes (154 MB) / 517275648 bytes (493 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 4156 (232736 bytes; 0 MB) allocated, 6 (336 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 2569 (143864 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Player Count: 1 / 8; [iq['allam326'/224, l='New World', x=297.32, y=176.00, z=643.13]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.