Tes – a negiya between the heads
Shevet Halevi (vol. 5, siman 8:9) brings a machalokes between the Mikdash M'at that pasels a tes that there is such a negiya between the heads, because they have turned it into a samech (and a shaylas tinok isn't accepted here), and the Mishnat Sofrim that allows fixing (scraping the negiya). Shevet Halevi is machmir as Mikdash M'at on this issue.
But the Mishnat Hasofer (Yalkut and Biur p. 59) disagrees, and holds iykar as Mishnat Sofrim, that this isn't an automatic shinuy tzura, and in a questionable situation, a shaylas tinok is accepted.
See Biur Hallacha 32:18 that a mem psucha that the chartom is touching the moshav, even the slightest ngiya "c'chut hasara" is pasul.
What is the difference between the negiya of the mem, and the negiya of the tes?