Drips and sags
If you see a drip forming while painting you can reduce the damage by spraying air at it through the spray gun. The air can hold it in position and dry it out reducing the rectification work later. The finish will be terrible and it will still need rectification. Never try to wipe out the drip - always leave it to dry before any rectification or else you could take out previous layers of paint.
Here's a nasty drip. It's beside a nasty rolled edge where a fair bit of paint went on at a sharp angle. I nearly got away with it, but for the bowl shaped depression just visible at the top of the photo which must have collected paint aimed at the rolled edge.
To avoid the drip in the first place I could have painted the inside of the rolled edge then let the paint dry before painting the rest of the panel.