A: No problem, mud the area, but make sure to prime/paint that mud area and let it dry prior to re-top coating everything or you'll get a flashing spot in the walls.
If you put a first coat on (even with some higher end paints) and the color is a deep base color (like a red or dark green etc) the paint will not cover in one coat (actually paint never really covers in one coat if you ask me). A second coat is needed but if you notice a few small holes you missed when you went around the first time and hit everything with the 5 minute mud (or 20 minute or joint compound or top coat etc whatever the situation calls for) All you need to do is hit that area with the mud, let it dry, prime it and then proceed to top coat everything.
More errors can show up after the first coat, because the reflective nature of the paint you are using as well as the color you are using may reveal more (or less for that matter) Generally, I've found that the marbling look you get after a first coat will reveal one or two holes (very minor ones) that may have been missed. Simple to fix and a perfect wall is the end result.
A couple of quick tips on sanding the spot. 1. Use a vacuum underneath every spot you apply mud. 2. Prime in a circular pattern a little further out from the actual mud spot.
From there just proceed with expertise and excellence.