Trying to understand postgres crash

Hi,

I'm running postgres 8.3.17 on a redhat linux mips derivative and I've
observed a postgres crash and the subsequent messages in my postgres
debug log.

2011-12-21 19:08:49 UTC - LOG: shutting down
2011-12-21 19:08:49 UTC - PANIC: concurrent transaction log activity
while database system is shutting down
2011-12-21 19:08:49 UTC - LOG: background writer process (PID 1423)
was terminated by signal 6: Aborted
2011-12-21 19:08:49 UTC - LOG: terminating any other active server processes

I'm having trouble understanding what could cause this. I've looked
through the postgres sources and I don't understand what is "shutting
down" since this happens on a live system where no stop has been
requested.

Also It happens very infrequently and I don't yet know how to reproduce it.

I'm happy to provide more information.

thanks in advance for the help!
-HH