world.cleanUp error at line 1102

found this when reloading, made clients time out...
does anyone here know what this is?

Code:

2011-09-22 07:12:06 [SEVERE] java.util.ConcurrentModificationException
2011-09-22 07:12:06 [SEVERE]     at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
2011-09-22 07:12:06 [SEVERE]     at java.util.ArrayList$Itr.next(Unknown Source)
2011-09-22 07:12:06 [SEVERE]     at net.minecraft.server.World.cleanUp(World.java:1102)
2011-09-22 07:12:06 [SEVERE]     at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:460)
2011-09-22 07:12:06 [SEVERE]     at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:374)
2011-09-22 07:12:06 [SEVERE]     at net.minecraft.server.ThreadServerApplication.run(SourceFile:417)
2011-09-22 07:12:06 [SEVERE] Unexpected exception
java.util.ConcurrentModificationException
    at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
    at java.util.ArrayList$Itr.next(Unknown Source)
    at net.minecraft.server.World.cleanUp(World.java:1102)
    at net.minecraft.server.MinecraftServer.h(MinecraftServer.java:460)
    at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:374)
    at net.minecraft.server.ThreadServerApplication.run(SourceFile:417)