java.util.ConcurrentModificationException

2012-06-29 23:05:06 [INFO] [BringBackTheEnd] All alone!
2012-06-29 23:05:06 [INFO] World setti_the_end is alone.
2012-06-29 23:05:06 [INFO] Unloaded spawn from memory for world: setti_the_end
2012-06-29 23:05:06 [INFO] Looking for dragon life in world setti_the_end...
2012-06-29 23:05:06 [SEVERE] java.util.ConcurrentModificationException
2012-06-29 23:05:06 [SEVERE] 	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
2012-06-29 23:05:06 [SEVERE] 	at java.util.ArrayList$Itr.next(ArrayList.java:791)
2012-06-29 23:05:06 [SEVERE] 	at net.minecraft.server.World.tickEntities(World.java:1174)
2012-06-29 23:05:06 [SEVERE] 	at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:560)
2012-06-29 23:05:06 [SEVERE] 	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:461)
2012-06-29 23:05:06 [SEVERE] 	at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
2012-06-29 23:05:06 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:819)
	at java.util.ArrayList$Itr.next(ArrayList.java:791)
	at net.minecraft.server.World.tickEntities(World.java:1174)
	at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:560)
	at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:461)
	at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)
2012-06-29 23:05:06 [INFO] No dragon was found!
2012-06-29 23:05:06 [INFO] Clearing and Regenerating chunks...
2012-06-29 23:05:06 [INFO] The End is being reset (setti_the_end).
2012-06-29 23:05:06 [INFO] Any attempts to access to that world will teleport to spawn!
2012-06-29 23:05:07 [INFO] Chunks marked for unloading.
2012-06-29 23:05:07 [INFO] Done with world setti_the_end.
2012-06-29 23:05:07 [INFO] The End (setti_the_end) has been reset!
2012-06-29 23:05:07 [INFO] You may now access it again.
2012-06-29 23:05:07 [INFO] [BringBackTheEnd] World setti_the_end done.