Nicks that can upset the rebua (square) of the tefillin
The tefillin must be square at every level, from the bottom of the titura until the top of the bayis. If the tefillin become nicked and lose the rebua at any level, they are pasul. The Mishna Berura is uncertain as to what size nick would disqualify tefillin. Even according to the stringent approach, a very tiny nick is not a problem.If the corners round out a bit, they remain kosher if they still look square. It is possible to rely on the chazaka and not inspect the rebua on a daily basis.
(סעיף לט, ס"ק קעט ו־קפא, וביה"ל ד"ה נתקלקל; ביאורים ומוספים דרשו, 216)