Trust in a Sofer
STa"M is one of the unique areas of Halacha in that complete trust is placed in the knowledge an integrity of the Sofer. I am certain that there are teshuvot about when a Sofer can and can not be trusted based on their personal conduct as well as actions they have taken in their profession.
E.g. The article posted by Nachman is a sad example of conduct that I'm sure (or at least would hope) would result in a dismissal of all confidence in the Sofer's integrity and yirat shamayim.
Based on this I have the following question, based on a real life event.
Say there is a Sofer whom you have never met nor seen written credentials, but who says he learnt by a well known rav and Sofer. This sofer seems to be one who is researching STa"M as he goes, there being no evidence of his having learnt (halachot at least) by a knowledgable rav.
After several years this sofer makes a well known and clear mistake. He wrote a lamed fully inside a kaf peshuta and to fix it he merely erases the rosh of the lamed and writes it outside the kaf. A blatant and obvious chak tochot. Upon being corrected by another person, this sofer says he looked it up and realized his mistake. He had not known about this beforehand.
Does this put a chashash on everything he has written?
If yes, how should it be dealt with? Who is responsible for dealing with it?
If no, why? What are the gedarim? Where do we draw the line?
Shevet HaLevi (I forgot the exact ref.) writes about sofrim who make the kotz tachtona of RT a sharp square, which acc to him is a misunderstanding of the chatam sofer. He says that this is not something that would cause a chashash on all their work.
But this case is clearly more serious. Does anyone have any insight or references?