Query bombed: why?

After about 25 minutes of running a query with a "where not exists
'correlated subquery'", I got a whole bunch of lines printing out: "Backend
sent D message without prior T".
Could someone give me an idea of what that means, and how to deal with it?
The query is on two tables, one having 465,266 rows with indexes on all
queried fields, the other has 70 rows with no index.  "Vacuum analyze" was
done prior to running the query.
My system administrator checked out the server, and saw nothing wrong.
I intended to send a copy of the output, but I cleared my screen by mistake:
sorry.  Please give me some indulgence, I am a beancounter, not a techie :-)