Downward 'drooping' movement of the paint film immediately after application, resulting in an uneven coating.
If the paint is still wet, immediately brush out or re-roll to redistribute the excess evenly. If the paint has dried, sand and reapply a new coat of top quality paint. Correct any unfavourable conditions; do not thin the paint, avoid humid and/or cool conditions, sand glossy surfaces. Paint should be applied at its recommended spread rate, avoid 'heaping on' the paint. Two coats of paint at the recommended spread rate is better than one heavy coat, which can lead to sagging. Consider removing doors to paint them supported horizontally.
Source: The Paint Quality Institute