By Rabbi Chaim Hillel Raskin – Rov of Anash in Petach Tikvah

The Mechilta says that if a mezuzah or tefillin were not written in order, they should be placed into geniza, as the possuk Vehayu hadevarim ha’eileh (“and these words shall be”) dictates that they must be written according to their original sequence in the Torah (kesidran).¹ Thus, Rishonim write that one cannot add missing letters to a mezuzah or tefillin (as one does for a Sefer Torah that is missing letters) for the added letters have not been written in order and are invalid.²

Shulchan Aruch rules that the parshiyos in tefillin and mezuzah, and the words contained in each parsha, must be written in order.³ The order the parshiyos of tefillin must be written — for both Rashi and Rabbeinu Tam’s tefillin — is: (1) Kadesh, (2) Vehaya ki yevi’acha, (3) Shema, (4) Vehaya im Shamo’a. Although the Taz quotes a lenient view on the parshiyos of Shema and Vehaya im Shamo’a written out of order,⁴ the Shach invalidates such a mezuzah, and the Alter Rebbe rules likewise about tefillin .⁵

Certain mistakes are considered writing from scratch when fixed (e.g., a letter that wasn’t written in the proper shape), and would constitute writing out of order. However, if the letter itself is written properly and the pesul is due to an external factor, it can often be fixed.

For example, a legible letter that touches the next letter is lacking hekef gevil (perimeter of blank parchment), but it can often be corrected by erasing some of the letter’s thickness. Similarly, if a part of the letter isn’t attached to the rest (e.g., yud of shin) but is read properly by a child who isn’t old enough to read, it can sometimes be fixed even out of order.⁶

If the missing letter is toward the end, one can erase all the subsequent letters and start again from there, unless Hashem’s name — which cannot be erased — is in that part.⁷

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From The Weekly Farbrengen by Merkaz Anash