BUG #8984: ERROR: t_xmin is uncommitted in tuple to be updated

The following bug has been logged on the website:

Bug reference:      8984
Logged by:          Marko Tiikkaja
Email address:      marko@joh.to
PostgreSQL version: 9.1.11
Operating system:   64-bit Ubuntu
Description:

Hi,

I just noticed we've had the error in subject happen around ten times,
starting from Thursday.  I don't see it having happened before that.  IIRC
that day we also switched to another synchronous slave.  The previous slave
had hot_standby_feedback=on, and the new one has it turned off.  Might just
be a coincidence.

All of the failing queries are inside PL/PgSQL functions, most of them are
SELECT .. FOR UPDATE or UPDATE against a table called "workers", but there
are a couple of instances against other tables as well.

Any ideas?  I'll be happy to provide more information, just let me know what
you need.