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Ok so i was in a world and i desided that i would try to find how high the Beacon goes so i went in creative and tried flying up to the top of it whilst looking down and i got above build height and kept going when this happend:
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Minecraft has crashed!
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Minecraft has stopped running because it encountered a problem; Exception in server tick loop
This error has been saved to C:\Users\eddie\AppData\Roaming\.minecraft\crash-reports\crash-2012-10-29_11.28.01-server.txt for your convenience. Please include a copy of this file if you report this crash to anyone.

--- BEGIN ERROR REPORT 55d3af08 --------
Generated 29/10/12 11:28
- Minecraft Version: 1.4.2
- Operating System: Windows 7 (amd64) version 6.1
- Java Version: 1.7.0_05, Oracle Corporation
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
- Memory: 375907048 bytes (358 MB) / 730202112 bytes (696 MB) up to 954466304 bytes (910 MB)
- JVM Flags: 2 total; -Xms512m -Xmx1024m
- AABB Pool Size: 2049 (114744 bytes; 0 MB) allocated, 39 (2184 bytes; 0 MB) used
- Is Modded: Very likely
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 1288 (72128 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
- Player Count: 1 / 8; [ih['pieman478'/71, l='Copy of Lan Game', x=328.53, y=312.72, z=1.51]]
- World Copy of Lan Game Entities: 0 total; []
- World Copy of Lan Game Players: 0 total; []
- World Copy of Lan Game Chunk Stats: ServerChunkCache: 0 Drop: 0
- Type: Integrated Server
java.lang.IllegalStateException: Entity is already tracked!
at hz.a(SourceFile:67)
at hz.a(SourceFile:48)
at ia.a(SourceFile:24)
at xe.a(SourceFile:786)
at ie.a(SourceFile:553)
at xe.d(SourceFile:778)
at xr.a(SourceFile:138)
at ie.b(SourceFile:106)
at net.minecraft.server.MinecraftServer.r(SourceFile:717)
at net.minecraft.server.MinecraftServer.q(SourceFile:661)
at bcb.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:595)
at fp.run(SourceFile:818)
--- END ERROR REPORT 7c83c0a1 ----------

Even though it states " Is Modded: Very likely" i have not modded it [as there arn't many mod packs availibule(yes im lazy) ]. Could some one please tell me wheather this is my fault or its th coding?