Error with a Vanilla build.

I keep getting this error with a core CraftBukkit server.
It happens after I die, for some reason there's an invisible mob that hits me when i return and then i die again and get kicked with this showing in the console.
I'm no Java expert but it looks like there's some sort of overlapping command going on with the EntityTracker which is causing the invisible mobs.

Code:

2011-04-08 18:42:34 [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)