Fret-board dandruff 

Had the chance to get one of my guitars on the bench a while back.  I actually bought this one used so the frets already had some wear (plus I am hard on frets when I play).  The white flakes on the second pic down are not dandruff, but because of the bad flat spots on some frets, when I ran my finger nail over the them they where so sharp it was cutting my nail.  Great as a nail file, not so great if you want a guitar to sound good and play well.
End result is I now play this guitar all a lot, It is made from Alder so sounds really good.  Its pretty much my go to when teaching clients. 
see finished picture down the page.