לזכות ר' אברהם יצחק בן אסתר
ר' יוסף עזרא בן אסתר
וכל בני ביתם
The gemara in Zevachim [2b] says that a get written with no particular woman in mind - סתמא - is pasul .The gemara wonders how we know this. It CAN'T be from the Mishna that says that a get is pasul if "שנכתב שלא לשם האשה" because that implies [as Rashi says] that it was written EXPLICITLY for another woman and not סתמא. Ultimately the gemara finds a source. [It is interesting that this discussion takes place in Zevachim and not earlier in Gittin.]
But the question remains - Why did the Mishna not use terminology to imply that סתמא is pasul, like שלא נכתב לשם האשה which would imply that any get not written for a specific woman is pasul??
The Chiddushei HaRim answers that there are cases where סתמא is kosher, such as where the husband himself wrote the get for "Leah" which just happens to be his wife's name but he wasn't thinking that "this is for Leah my wife". In such an instance, the get is kosher because it makes sense that he meant it for his wife Leah. The Mishna says that a get is pasul when שנכתב שלא לשם האשה i.e. it was explicitly for another woman, to teach that even if the man's wife is Leah [and thus סתמא is kosher], if he had ANOTHER Leah in mind - the get is invalid.
