From: Rabbi Moshe Sofer
To: Rabbi Aryeh Yehuda Leib
A sofer corrected a kaf sofis into a hei by scraping and adding a leg, potentially constituting chok tochos, but the location of the corrected letter could not be found. The Chasam Sofer rules leniently that the Sefer Torah may continue to be used, combining multiple lenient opinions.
Source: Shu"t Chasam Sofer OC §178