index corruption on composite primary key indexes

I've noticed what appears to be index corruption on composite primary
key indexes during my testing. Data deletes, updates, and inserts are
applied from delta data that is loaded into temporary tables. The
duplicate key error occurs at different points in time and isn't
isolated to any single table, although all affected tables have a
composite primary key index. For example, if machine1 and machine2 both
start off from the same starting state, then machine1 might fail on
delta 100 while machine2 won't fail until delta 125. The error in the
log is "duplicate key value violates unique constraint" with a reference
to the composite primary key index. The issue occurs on different
machines, so that would seemingly rule out hardware failure. The fact
that it only happens on tables with composite primary keys is highly
suspicious. It occurs reproducibly enough that it seems to be a pgsql
bug, or maybe a pgsql running on a certain set of hardware + software.

Some info on the platform I'm using:

PostgreSQL 8.4.4

Red Hat 4.1.2-44

Linux 2.6.18-128.el5 #1 SMP Wed Dec 17 11:41:38 EST 2008 x86_64 x86_64
x86_64 GNU/Linux

Intel(R) Xeon(R) CPU E5420 @ 2.50GHz

Does anyone know if this a known issue? Any help would be much
appreciated.