"Could not open relation with OID x" while deleting a row

Hi,

postgres just started to report this error yesterday when I ran a user
function which deletes rows in a lot of tables.
I get an error:

"Could not open relation with OID 18789"

The function looks like this:

CREATE OR REPLACE FUNCTION delete_photo(int8)
RETURNS bool AS
$BODY$
DECLARE
gid RECORD;
photo RECORD;
BEGIN
select into photo * from photos where "ID"=$1;
FOR gid in select * from tag where "ObjectID"=$1 LOOP
UPDATE tags_history SET "TagCount"="TagCount"-1 WHERE
"Tag"=gid."Tag" AND "Date"=current_date;
IF NOT FOUND THEN
INSERT INTO tag_history ("Tag","TagCount") VALUES
(gid."Tag",-1);
END IF;
END LOOP;

DELETE FROM comment WHERE "OwnerID"=$1;
DELETE FROM tag WHERE "ObjectID"=$1 and "Type"='photos';
DELETE FROM favorite WHERE "ObjectID"=$1 AND "Type"='photos';

FOR gid in select * from rels where "ObjectID"=$1 AND "Role"='pool' LOOP
UPDATE poster SET "ObjectCount"="ObjectCount"-1 WHERE
"GroupID"=gid."OwnerID" AND "Author"=photo."Author";
END LOOP;
DELETE FROM rels WHERE "ObjectID"=$1 AND "Role"='pool';
DELETE FROM rest WHERE "ObjectID"=$1 AND "Type"='photos';
DELETE FROM rest3 WHERE "ObjectID"=$1 AND "Type"='photos';
DELETE FROM photos WHERE "ID"=$1; <<<<-------ERROR IS RAISED HERE

RETURN True;

END;

The weird thing is that if I move the last delete statement to the second
row, the function starts to work.

Why? What was going on here?

Thanks, Sebastjan