- ModificationException error causes Server Crash.

This is an error that causes our server to crash randomly.
I'd like to know if you may know the cause of this issue?

CB: 1060

Code:

2011-09-02 20:05:51 [SEVERE] java.util.ConcurrentModificationException
2011-09-02 20:05:51 [SEVERE]     at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:782)
2011-09-02 20:05:51 [SEVERE]     at java.util.ArrayList$Itr.next(ArrayList.java:754)
2011-09-02 20:05:51 [SEVERE]     at net.minecraft.server.World.cleanUp(World.java:1129)
2011-09-02 20:05:51 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:450)
2011-09-02 20:05:51 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:363)
2011-09-02 20:05:51 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-09-02 20:05:51 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:782)
    at java.util.ArrayList$Itr.next(ArrayList.java:754)
    at net.minecraft.server.World.cleanUp(World.java:1129)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:450)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:363)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:422)
2011-09-02 20:06:17 [INFO] Connection reset