It is inevitable, skim long and hard enough and your board is going to get some dings.
Dings are harmless if repaired in a timely matter.
However if you don't fix them they just get worse and worse and will make you board fall apart in short order. In general anytime water can get under the fiberglass and into the foam you have a problem. The water will help deteriorate the foam and the size of the ding will grow. To keep this from happening make sure and repair your board. I probably haven't deterred you however and you still want to fix it yourself.