When a certain user logs in (or anyone approaches certain coordinates, the game crashes. The symptom is massive lag plus chunks not loading. There is a stack trace being generated.

---- Minecraft Crash Report ----
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Time: 3/16/13 4:38 PM
Description: Loading entity NBT

java.lang.NullPointerException
at net.minecraft.server.v1_4_R1.EntityItemFrame.a(SourceFile:64)
at net.minecraft.server.v1_4_R1.EntityItemFrame.a(SourceFile:93)
at net.minecraft.server.v1_4_R1.Entity.e(Entity.java:1160)
at net.minecraft.server.v1_4_R1.EntityTypes.a(SourceFile:122)
at net.minecraft.server.v1_4_R1.ChunkRegionLoader.loadEntities(ChunkRegionLoader.java:333)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:47)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:13)
at org.bukkit.craftbukkit.v1_4_R1.util.AsynchronousExecutor$Task.finish(AsynchronousExecutor.java:179)
at org.bukkit.craftbukkit.v1_4_R1.util.AsynchronousExecutor.finishActive(AsynchronousExecutor.java:287)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOExecutor.tick(ChunkIOExecutor.java:30)
at net.minecraft.server.v1_4_R1.MinecraftServer.r(MinecraftServer.java:537)
at net.minecraft.server.v1_4_R1.DedicatedServer.r(DedicatedServer.java:224)
at net.minecraft.server.v1_4_R1.MinecraftServer.q(MinecraftServer.java:494)
at net.minecraft.server.v1_4_R1.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.v1_4_R1.ThreadServerApplication.run(SourceFile:849)

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

– Head –
Stacktrace:
at net.minecraft.server.v1_4_R1.EntityItemFrame.a(SourceFile:64)
at net.minecraft.server.v1_4_R1.EntityItemFrame.a(SourceFile:93)

– Entity being loaded –
Details:
Entity Type: ItemFrame (net.minecraft.server.v1_4_R1.EntityItemFrame)
Entity ID: 38666
Name: entity.ItemFrame.name
Exact location: -9979.50, 67.50, -49977.06
Block location: World: (-9980,67,-49978), Chunk: (at 4,4,6 in -624,-3124; contains blocks -9984,0,-49984 to -9969,255,-49969), Region: (-20,-98; contains chunks -640,-3136 to -609,-3105, blocks -10240,0,-50176 to -9729,255,-49665)
Momentum: 0.00, 0.00, 0.00
Stacktrace:
at net.minecraft.server.v1_4_R1.Entity.e(Entity.java:1160)
at net.minecraft.server.v1_4_R1.EntityTypes.a(SourceFile:122)
at net.minecraft.server.v1_4_R1.ChunkRegionLoader.loadEntities(ChunkRegionLoader.java:333)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:47)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOProvider.callStage2(ChunkIOProvider.java:13)
at org.bukkit.craftbukkit.v1_4_R1.util.AsynchronousExecutor$Task.finish(AsynchronousExecutor.java:179)
at org.bukkit.craftbukkit.v1_4_R1.util.AsynchronousExecutor.finishActive(AsynchronousExecutor.java:287)
at org.bukkit.craftbukkit.v1_4_R1.chunkio.ChunkIOExecutor.tick(ChunkIOExecutor.java:30)
at net.minecraft.server.v1_4_R1.MinecraftServer.r(MinecraftServer.java:537)
at net.minecraft.server.v1_4_R1.DedicatedServer.r(DedicatedServer.java:224)
at net.minecraft.server.v1_4_R1.MinecraftServer.q(MinecraftServer.java:494)
at net.minecraft.server.v1_4_R1.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.v1_4_R1.ThreadServerApplication.run(SourceFile:849)