Most people are aware that the halacha requires that each letter in a mezuzah be written perfectly. Also, it must not be written by erasing. For example, if in error the Sofer wrote a beis instead of a kof, he might be tempted to save time by erasing the foot of the beis so that the kof would remain. This however, is writing by erasing and the mezuzah would not be considered kosher.
This law also applies if a drop of ink splashes onto a letter. Even if the ink can easily be cleaned away leaving a letter intact, the law of writing by erasing may have been violated.
So what this all means is that even if a mezuzah looks fine, it might actually be posul. That is why mezuzahs need to be purchased from a certified scribe who has been tested and is aware of the many laws, and most importantly, has a fear of G-d. Who knew that what you can’t see on a parchment is as equally important as what you can. Who knew?