To follow the last post, try this. Keep sanding all the way until you sand thru the paint, the very last bit of paint will be the paint right under that run you had. The paint next to that will disappear and you'll hit primer BEFORE that paint UNDER the run. That is the PROOF that the surrounding paint cut faster than the run. Remember, the surrounding paint is three coats or so right? Maybe three mils thickness. The run on the other hand is more like six or seven coats, maybe more. More like seven or eight mils. If sanded across the two, you will sand thru the three mils before you sand thru the eight.
I have to tell you, I was taught by the head painter at work to shave off a run with a single edge razor, it is AMAZING. I am not talking "shaving" like you would shave a sticker off glass or something with the blade laying on the glass and getting under it. I am talking "raking" the blade with it at a perfect right angle to the surface! It blew me away, but it works!
I'll have to take a few photos and make up a "tip of the day".