My bits moved right off the end of the world...

This message appeared from the backend when running vacuum 
analyze on our live data. I remember seeing a posting 
regarding this sometime back but no longer have this message. 

The version of postgres is 6.5.1 on RH6. There is more than 
sufficient disk space left, and postmaster does not seem
to be running out of memory.

Any help would be much appreciated.
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Regards
Theo