UPDATE statement with syntax error doesn't raise a warning?

I've ran an update statement like this (obviously wrong, I know!):

update my_table
set boolean_field = true AND
my_notes = 'something'
where id in
(select id from my_table order by random() limit 4000);

in my psql client and I had a "UPDATE 4000" result but, correctly, anything
was changed in my_table.

So... why pg didn't raise a warning about syntax issue?

Thank you!

Mirko

P.S.
PostgreSQL 8.3.8 on i686-pc-linux-gnu, compiled by GCC gcc (Debian
4.3.2-1.1) 4.3.2
psql 8.3.8 (Ubuntu package)