"Already being Tracked" error right before being kicked off.

Everyone on my server gets kicked off at random times with a "internal server error".
Not sure where to look. Been getting this since 1.4 came out.

Code:

2011-04-07 19:40:17 [WARNING] Failed to handle packet: java.lang.IllegalStateException: Entity is already tracked!
java.lang.IllegalStateException: Entity is already tracked!
    at net.minecraft.server.EntityTracker.a(EntityTracker.java:72)
    at net.minecraft.server.EntityTracker.a(EntityTracker.java:62)
    at net.minecraft.server.EntityTracker.a(EntityTracker.java:51)
    at net.minecraft.server.WorldManager.a(WorldManager.java:17)
    at net.minecraft.server.World.b(World.java:750)
    at net.minecraft.server.WorldServer.b(WorldServer.java:94)
    at net.minecraft.server.World.a(World.java:1590)
    at net.minecraft.server.Chunk.d(Chunk.java:470)
    at net.minecraft.server.ChunkProviderServer.c(ChunkProviderServer.java:75)
    at net.minecraft.server.PlayerInstance.<init>(PlayerInstance.java:33)
    at net.minecraft.server.PlayerManager.a(PlayerManager.java:37)
    at net.minecraft.server.PlayerManager.c(PlayerManager.java:141)
    at net.minecraft.server.ServerConfigurationManager.b(ServerConfigurationManager.java:111)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:268)
    at net.minecraft.server.Packet10Flying.a(SourceFile:126)
    at net.minecraft.server.NetworkManager.a(NetworkManager.java:198)
    at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:72)
    at net.minecraft.server.NetworkListenThread.a(SourceFile:100)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:368)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:283)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:375)

 

I've been seeing this a few times myself, although my crashes of late have no indication as to what caused it.