java.util.ConcurrentModificationException in FSNamesystem.chooseTargets

2007-01-26 01:14:37,509 INFO org.apache.hadoop.ipc.Server: IPC Server handler 7 on 8020 call error: java.io.IOException: java.util.ConcurrentModificationException
java.io.IOException: java.util.ConcurrentModificationException
at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:449)
at java.util.AbstractList$Itr.next(AbstractList.java:420)
at org.apache.hadoop.dfs.FSNamesystem.chooseTargets(FSNamesystem.java:2282)
at org.apache.hadoop.dfs.FSNamesystem.startFile(FSNamesystem.java:484)
at org.apache.hadoop.dfs.NameNode.create(NameNode.java:238)
at sun.reflect.GeneratedMethodAccessor60.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:337)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:538)

Not sure if it's related, but this exception happend when namenode was replicating many blocks.