Fixing Airbrush problems

If you are using good, properly thinned, clean paint, this shouldn't happen. You should always "open up" away from the subject to confirm you have good flow before you start, & if you still are not confident, then do the same whenever "open up" after stopping the paint flow.

Stuff does happen though, but if you keep a keen eye on the paint as it is laying down & also listen to your brush, you should be able to stop the flow of paint before much damage is done. Regards the damage, if it's towards the end of a job I usually let it dry, smooth it down with fine paper if required & then do a "touch up" - think about it, automotive body shops make a living by doing this with overgrown airbrushes.

Or do we do our thing with shrunken spray guns? 

Fine sandpaper and redoe. I usually use 600 wet or dry in a case like that unless it is a really bad, thick blotch and then I have to use something coarser.  If I am doing a plane, I try to keep all sanding and retouching within the panel(s) on which the problem occurred.

If it is a ship or car with no natural panel demarkation, I try to feather it out and then polish to blend in with the old paint, using very fine polishing material.  Micro Mark and some other places now carry some polishing kits with sponge/foam with VERY fine grit.