Sorry if this is a duplicate report. This is the first time I've felt like I needed to report a bug.

Put the summary of the bug you're having here

I downloaded 13W02b and installed it into my minecraft folder. I skipped the update becuase I had 13W02b. I decided to copy a world that I enjoy to play in. It was in hardcore mode of survival. I spawned near some sheep and started to attack them to get wool. Right when I think I was about to kill one, Minecraft crashed. I tried to replicate it, but it did not happen again.

Here is the crash text.

Minecraft has crashed!
----------------------

Minecraft has stopped running because it encountered a problem; Ticking memory connection

A full error report has been saved to C:\Users\Alonzo\AppData\Roaming\.minecraft\crash-reports\crash-2013-01-13_13.17.34-server.txt - Please include a copy of that file (Not this screen!) if you report this crash to anyone; without it, they will not be able to help fix the crash

— BEGIN ERROR REPORT 65e476a9 --------
Full report at:
C:\Users\Alonzo\AppData\Roaming\.minecraft\crash-reports\crash-2013-01-13_13.17.34-server.txt
Please show that file to Mojang, NOT just this screen!

Generated 1/13/13 1:17 PM

– System Details –
Details:
Minecraft Version: 13w02b
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_07, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 349907224 bytes (333 MB) / 514523136 bytes (490 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 8383 (469448 bytes; 0 MB) allocated, 8383 (469448 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: 1177 (65912 bytes; 0 MB) allocated, 1035 (57960 bytes; 0 MB) used
Player Count: 1 / 8; [iq['asoban'/216, l='Headstrong2', x=9.61, y=63.00, z=216.72]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

java.lang.IllegalStateException: Entity is already tracked!
at ii.a(SourceFile:69)
at ii.a(SourceFile:47)
at ij.a(SourceFile:24)
at yn.a(SourceFile:843)
at in.a(SourceFile:610)
at yn.d(SourceFile:835)
at lv.a(SourceFile:1073)
at pk.a(SourceFile:116)
at mj.a(SourceFile:830)
at mj.a(SourceFile:712)
at pd.a(SourceFile:122)
at rh.q(SourceFile:843)
at iv.a(SourceFile:550)
at dp.a(SourceFile:36)
at cf.b(SourceFile:55)
at iv.d(SourceFile:57)
at iw.b(SourceFile:39)
at bfg.b(SourceFile:84)
at net.minecraft.server.MinecraftServer.r(SourceFile:763)
at net.minecraft.server.MinecraftServer.q(SourceFile:680)
at bfd.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:614)
at fy.run(SourceFile:850)
— END ERROR REPORT c640628a ----------