Who is responsible to repair the parshios?
The sofer must pay careful attention to the proper spelling of words and the formation of letters as handed down through the mesora. Improperly formed, missing or extra letters will disqualify the parsha. If such writing is used in tefillin, the wearer will lose out on the mitzva and recite daily berachos levatalah. After completing the writing, the sofer must proofread it two or three times to ensure accuracy. A sofer may not release a parsha until he has inspected it properly. Nonetheless, it is prudent for the consumer to have the parsha rechecked by a professional and even have it checked by a computer.
(סעיף כ-כא וס"ק קא; ביאורים ומוספים דרשו, 150)