Bogus "Non-functional update" notices

With fairly current sources (last cvs update on 7/20), I am seeing
occasional occurrences of
NOTICE: Non-functional update, only first update is performed
I think this is a bug. The update commands that are triggering this
message *are* getting executed. I looked at the sources and couldn't
even understand what condition was being tested to generate the message.
The source code looks like it's trying to disallow more than one update
to the same tuple within a transaction, which is so silly that I have to
be misreading it...

Here is an example trace of my application's interaction with the
server:

// Tuple 134537 is created here:

QUERY: BEGIN TRANSACTION; LOCK marketorderhistory
RESULT: DELETE 0
// several other tuples inserted or updated in this transaction
QUERY: INSERT INTO marketorderhistory (accountID, instrumentID, orderType, numContracts, orderTime, simStatus, realStatus, sequenceNo, orderPrice, orderDivisor, ifDonePrice) VALUES(5, 62, 'S', 5, '1998-05-20 15:20:00 GMT', 'P', '-', nextval('marketorderhistory_Seq'), 11969, 100, 11849)
RESULT: INSERT 134537 1
QUERY: END TRANSACTION; NOTIFY marketorderhistory
RESULT: NOTIFY

// many transactions later, the app wants to update this tuple:

QUERY: BEGIN TRANSACTION; LOCK marketorderhistory
RESULT: DELETE 0
QUERY: UPDATE marketorderhistory SET completionTime = '1998-05-21 15:20:00 GMT' WHERE oid = 134537::oid AND completionTime IS NULL; UPDATE marketorderhistory SET simStatus = 'X', sequenceNo = nextval('marketorderhistory_Seq') WHERE oid = 134537::oid
NOTICE: Non-functional update, only first update is performed
RESULT: UPDATE 1
// a couple other tuples inserted or updated
QUERY: END TRANSACTION; NOTIFY marketorderhistory
RESULT: NOTIFY

External inspection verifies that both updates did take effect.

The thing that's weird is that this only happens occasionally, say about
twice out of every thousand essentially identical updates. I don't
know enough about the backend innards to have much chance of figuring
out what's going on. Any ideas? Is anyone else even seeing this?

regards, tom lane