When creating a skin, people will often forget to fill in certain parts of it (certain sides of arms, legs, etc.) and publish the skin without knowing. If a certain part of the skin isn't filled out when the creator decides to publish the skin, a message could pop up saying something like "Not all parts are filled in (gives a list of parts to check, or tells you what isn't filled in), are you sure you want to publish this skin?" Obviously there could be a much more thought out message than that, but that's just an example. I believe that this could help a lot of beginners who forgot to fill in the inner parts of a skin (what I mean by "inner parts" of a skin is shown below,) and even more experienced artists who have made a small mistake.

To clarify, by inner parts of a skin, I mean the spaces between the torso and arms, both legs, the underside of the head, etc.