An incident happened today where someone whom I checked their Tefillin around 16 months ago had them checked just now again by another magiah sta"m from out of town.
The person only in a side conversation (he was very upset) revealed to me that the tefillin I checked were pasul (and now fixed by the other magiah) since he was told after the checking that the "tifrim were not ribua" on the bottom.
This was shocking to me since I am very careful in measuring the tefarim that they are kosher so I called the other magiah to find out the exact story.
When I told him what the person told me, he immediately apologized for not being clear to the client that there was some warping which caused the tifrin to move a little which he was sure it was not my fault and also was not sure if they were mamash posul. He than said he should of been more careful and clear with the client.
The lesson from this story is that a magiah must be very careful on his comments after a checking.
Keep in mind if the potential pasul is a machlokes among Poskim in the final ruling. Also consider if the potential pasul was really the fault of the previous magiah or if it occurred naturally with time.
When in doubt lichora give the previous magiah the benefit of the doubt האי כלל נקיט ביד' אין מחזיקים
Many times we could cause a nezek to another sofer/magiah (and sometimes even a reckless nezek)just by not being enough sensitive in this issue.
I also recommend that if you feel it is necessary to tell the client that the sta"m was pasul and now fixed to take a picture for proof.