That has happened to my snakes before, it happens with a shed. It seems like the top layer of the new scale comes off with the old- if you look at the shed, you will see an area of pigmentation matching that spot on the head. I don't know what causes it, but it hasn't caused mine any problems and is repaired with the next shed. I'd like to know myself how to prevent this if anyone knows. Maybe it's a humidity issue?