A customer came to me last week to check his tefillin. I opened them and discovered that he Rabainu Tam Tefillin. This means that he never put on Rashi Tefillin until he barowwed a pair from me to use while I was checking his. He had been using these since he was Bar Mitzvah (13 years old).
People often ask me if they really need to check their tefillin, especially since the halachah is that if they are kosher, and they are worn everyday, that they need not be checked. I answer them that I see so many unbelievable things when I check supposedly kosher tefillin that it is impossible for me to tell them they don’t need to check them. I tell them that after I check them I can tell them that they don’t need to be checked for a long time. Of course, I tell them that if they trust the sofer who worked on them last, they may feel comfortable not checking them, but if I don’t know that sofer, I can’t tell them to rely on him.