It says in the Mishnah in Gitten (See here Daf 87b)
That if an eid (witness) signs on the Get "ben ish ploni eid" then it's kosher. Meaning instead of writing for example his full name "Moshe ben Yaakov" he writes simply "ben Yaakov". Still the get is kosher.
However I don't understand how this works.
What about if there is another person named "Yaakov" in the city that has boys, how do we know that this "ben Yaakov" is really the one that signed? And if you'll say there is only one "Yaakov" in the whole city, what about if Moshe ben Yaakov has other brothers (Yitzchak ben Yaakov, Yehoshua ben Yaakov, etc, etc.) How do we know who the person that signed is since they are really all technically "ben Yaakov"?