I used the function Zend_Db_Adapter_Abstract::fetchCol() to return all the values in a single column. When it called Zend_Db_Statement::fetchAll() the result set was incomplete. I traced the problem to this while loop (Zend/Db/Statement.php, starting around line 328):

            while ($val = $this->fetchColumn($col)) {
                $data[] = $val;
            }

When the column value was null or 0, the while loop ended, even though it contained only a partial list of the values.

I fixed it on my local instance by making the following change:

            while (($val = $this->fetchColumn($col)) !== false) {
                $data[] = $val;
            }

I leave it to you to decide if this is the best approach. Fixed it for me though. I'm using release 1.7.7.