current_user changes immediately after login

Hi list,
I have a strange problem: When I connect to one of my databases, the
current_user immediatly changes without any interaction from my side.
This is what I do:

chschroe(at)db1:~> psql -h db2 testdb
Welcome to psql 8.2.5, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help with psql commands
\g or terminate with semicolon to execute query
\q to quit

testdb=> select session_user;
session_user
--------------
chschroe
(1 row)

testdb=> select current_user;
current_user
--------------
itstaff
(1 row)

How can this happen? The consequence of this is that I cannot access the
tables because the role "itstaff" has no privileges. "reset role" does
not have any effect. After "set role none" the current_user is equal to
the session_user and the tables are accessible again.

This problem does only occur with this database. Do you have any
explanation?

Regards,
Christian

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