Sometimes we are not sure if a letter is kosher. Since we know what it is supposed to be we cannot rely on our own judgment. In cases like this sometimes the halacha dictates that we ask a child who is neither a fool or a chochom (wise person). This means that the child should know how to read the letters but not understand what he is reading. In a case where there is a 50% chance that the child will read either one of two possibilities we can rely on his ‘psak’ (decision). If you look at the vov in the picture you will see that it is touching the lamed underneath. When we covered up the bottom part of the Lamed (which was necessary) the child read it as a nun. If the sofer would scrape away the part that was touching he would be turning a nun into a vov out of order, rendering the mezuza posel. By the way this would also be chok tochos (creating a letter by carving with a knife instead of by writing with ink).