I heard a kasha today from R' Shammai Gross that I wanted to share with the olam.
The tzura of a mem (both pasucha and stumah) is that of a chof and vav (only difference is the bottom) if that's the case why is it if someone writes a Mem-Pasucha with a chof and then a line (not with a tzurah of a vav) that it's pasul and can't add onto it (see Mishnas HaSofer) while by a Mem-Stumah if someone wrote a box and didn't put a small kav on the top left he would be allowed to add it on b'dieved. In both cases you are being m'sakain the "vav" so what's the difference
(I hope I have said over the kasha in the right way. Any mistake is mine and in a lack of understanding perhaps of what he was asking...)