BUG #4120: ERROR: cache lookup failed for function 0

The following bug has been logged online:

Bug reference:      4120
Logged by:          Pedro Gimeno
Email address:      pgsql-002@personal.formauri.es
PostgreSQL version: 8.2.5
Operating system:   Debian Etch + Backports
Description:        ERROR: cache lookup failed for function 0
Details:

I have an application running under Windows (tried both with XP and 2000)
which when run, gets the server to a point in which every comparison of
varchar < varchar gives the above error:

postgres=# select ''::varchar < ''::varchar;
ERROR:  cache lookup failed for function 0

It doesn't seem to matter where it's run: that last command was run in the
Debian box, so it doesn't look like a client-side problem. However I've been
unable to force that state by any other means but running my application.

Changing the direction of the comparison makes things work again, i.e. if I
write "SELECT b > a" instead of "SELECT a < b" it works.

Is there anything I can do to help tracking down this problem?

-- Pedro Gimeno