It happened to me again last Friday. I found a missing word in someone's tefillin. The owner of the tefillin is about 55, he bought the tefillin about twenty years ago and had them checked only once since then.
The first question I always get asked in these situations is "Was I yotzei mitzvas tefillin for the past 20 years or not?" When it's a psul like a small negiya, there is yesh lismoch. Likewise with nifsakim, spacing and other potential errors, you can find what to rely on for the past. But it's hard to sugar coat a spelling mistake.
There is one teshuvah from the Har Tzvi that's seems to imply you are yotzei if your intent was there, even if the tefillin were possul. Is this a limmud zchus or actually the halacha? I never got it clear. I like to make people feel better, but I need to base it on something legit.
If anyone has teshuvos or mekoros or advice on this inyan I'd like to please know about it.