Bug in psql - Postgresql 7.3.1?

All,

I'm not certain if what I'm trying to do is legal, but if I execute a
statement like:

UPDATE my_table SET field1='new_value' AND SET field2='different_value'
WHERE my_table_id = 'key';

in psql, it reports that it has successfully updated one record.
However, the record does not appear to be updated when I subsequently
issue a SELECT for that record. I'll buy the fact that I may be using
illegal syntax - I'm not SQL expert, but the update says it worked when
in fact it did not. Should this have worked or should it have reported
an error?

For record, this is Postgresql 7.3.1 on Solaris 9. Problem originally
seen during a php4 script, but has been duplicated on the psql command
line.

TIA,

John