There are several differences between the 2 drawings of a head below...can you spot them?
The obvious (?) answer is that the top one is the finished version, & the lower one is the unfinished version -- I didn't realize that it was unfinished when I scanned it in. The differences are many, but basically the hair has been textured more, the facial features have been better defined & made more dramatic, the background has been varied a bit, & so on. I tend to jump around from area to area when fine-tuning a piece, working by instinct as much as anything.