attempted to lock invisible tuple - PG 8.4.1

I'm running our products test suite against PostgreSQL 8.4.1. The test
suite runs fine against 8.3.7.

With 8.4.1, some of our tests are failing with the exception
'attempted to lock invisible tuple'. The failures are repeatable -
they crash every time at the same point. They crash no matter if they
are being run in isolation or as part of the larger test suite.

Anyone know what we could be doing that triggers that? Looking at our
statement logs we don't seem to be doing anything unusual. The failing
tests I've checked are running under SERIALIZABLE isolation level, and
the database will have been recreated a few instants ago using
'createdb --template test_template_db'.

One of the statement logs is at http://paste.ubuntu.com/285983/ - I
can't see anything unusual
going on but it might help diagnose the problem.

--
Stuart Bishop <stuart(at)stuartbishop(dot)net>
http://www.stuartbishop.net/