I see quite a lot of these errors in the logs: Error - Call to a member function rowCount() on boolean in /var/www/app/fuel/core/classes/database/pdo/connection.php on line 272
and other places where rowCount() occure.
Now its is quite difficult to reproduce that error but in general when you get your query running and kill the process on the DB, this is what happens.
It shouldn't return false, the PDO driver sets the error mode to PDO::ERRMODE_EXCEPTION to throw exceptions on errors instead of silently fail. Did you override the PDO attributes in your config?