Postgresql 8.3.3 refuces to start after increasing shared_buffers

Setting
shared_buffers = 28MB

makes the startup script say
/etc/init.d/postgresql-8.3 restart
 * Service postgresql-8.3 starting
 * Starting PostgreSQL ...
waiting for server to
start...............................................................could
not start server                                [ !! ]
 * The pid-file doesn't exist but pg_ctl reported a running server.
 * Please check whether there is another server running on the same
port or read the log-file.
     [ !! ]
 * ERROR:  postgresql-8.3 failed to start

Reducing it to 27 MB makes it start right away.
What is this strange thing?

The postgresql.conf file is the one that was installed by default by
my linux distribution and I've only changed the logging (stderr
logging).