I was flying around y:300 looking at the top of a Beacon Beam and the game randomly froze for a few seconds then crashed when I was moving around/in the beam. It was NOT an out of memory error.

---- Minecraft Crash Report ----
// This is a token for 1 free hug. Redeem at your nearest Mojangsta: _HUG~

Time: 12/23/12 11:50 AM
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: LAN Building World
All players: 1 total; [iq['Avantir_Yihn'/624804, l='LAN Building World', x=-22.38, y=318.72, z=134.30]]
Chunk stats: ServerChunkCache: 529 Drop: 0
Level seed: -5851164172971180867
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: World: (-39,64,190), Chunk: (at 9,4,14 in -3,11; contains blocks -48,0,176 to -33,255,191), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
Level time: 529619 game time, 13967 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 3353 (now: false), thunder time: 3353 (now: false)
Level game mode: Game mode: survival (ID 0). 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: 1.4.6
Operating System: Mac OS X (i386) version 10.7.5
Java Version: 1.6.0_37, Apple Inc.
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Apple Inc.
Memory: 206570232 bytes (197 MB) / 550305792 bytes (524 MB) up to 1070399488 bytes (1020 MB)
JVM Flags: 3 total; -Xbootclasspath/a:/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/Resources/LauncherSupport.jar -Xms512M -Xmx1024M
AABB Pool Size: 2973 (166488 bytes; 0 MB) allocated, 2 (112 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: 2416 (135296 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
Player Count: 1 / 8; [iq['Avantir_Yihn'/624804, l='LAN Building World', x=-22.38, y=318.72, z=134.30]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.