Hallo,

  I think, that I found reproducible bug:

% sql template1
Welcome to the POSTGRESQL interactive sql monitor:
  Please read the file COPYRIGHT for copyright terms of POSTGRESQL
[PostgreSQL 6.5.0 on i586-pc-linux-gnu, compiled by gcc egcs-2.91.66]

   type \? for help on slash commands
   type \q to quit
   type \g or terminate with semicolon to execute query
 You are currently connected to the database: template1

template1=> insert into pg_group values ('dummies', 501, '{503}');
INSERT 18784 1
template1=> select * from pg_user where usename = 'dummy';
usename|usesysid|usecreatedb|usetrace|usesuper|usecatupd|passwd  |valuntil
-------+--------+-----------+--------+--------+---------+--------+--------
dummy  |     503|f          |t       |f       |t        |********|        
(1 row)

template1=> create table tab ( i int );
CREATE
template1=> \z tab
Database    = template1
 +----------+--------------------------+
 | Relation | Grant/Revoke Permissions |
 +----------+--------------------------+
 | tab      |                          |
 +----------+--------------------------+
template1=> grant all on tab to group dummies;
CHANGE
template1=> \z tab
Database    = template1
 +----------+----------------------------+
 | Relation | Grant/Revoke Permissions   |
 +----------+----------------------------+
 | tab      | {"=","group dummies=arwR"} |
 +----------+----------------------------+
template1=> delete from pg_group;
DELETE 1
template1=> \z tab
NOTICE:  get_groname: group 501 not found
pqReadData() -- backend closed the channel unexpectedly.
	This probably means the backend terminated abnormally
	before or while processing the request.



And in log is:
PGSQL: NOTICE:  get_groname: group 501 not found 
PGSQL: NOTICE:  Message from PostgreSQL backend: 
PGSQL: ^IThe Postmaster has informed me that some other backend died
abnormally and possibly corrupted shared memory.
PGSQL: ^II have rolled back the current transaction and am going to terminate your database system connection and exit. 
PGSQL: ^IPlease reconnect to the database system and repeat your query. 
 

This look like that \z causes (if group doesn't more exist) database crash
... and I hate crashes ...
-- 
* David Sauer, student of Czech Technical University
* electronic mail: davids@orfinet.cz (mime compatible)

21 Jun 1999 08:24:32 +0200
David Sauer <davids@orfinet.cz>
crash if group doesn't exist (postgres 6.5, linux 2.2.10, rh 6.0)