data node throwing IOException "Unknown opcode for incoming data stream"

[[EMAIL PROTECTED]]

  hey guys,
  my data nodes are logging this exception occasionally. i'm using 
hadoop-0.12.3.
  
2007-08-06 09:20:00,185 ERROR org.apache.hadoop.dfs.DataNode: DataXCeiver
java.io.IOException: Unknown opcode for incoming data stream
 at org.apache.hadoop.dfs.DataNode$DataXceiver.run(DataNode.java:605)
 at java.lang.Thread.run(Thread.java:595)
  what could possibly be causing an unknown opcode ?
  
  thanks,
  mw
  
  
       
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