Gittin, Chapter 3, Mishnah 1. This mishnah deals with the fundamental requirement of lishmah: a get must be written for the sake of divorcing this particular man from this particular woman. The mishnah presents four levels, each one a step beyond the previous, in all of which this condition is missing and the get is invalid.
"Kol get shenichtav shelo l'shum ishah - pasul": - Any get that was written not for the sake of a particular woman is invalid.
A get that was not written for the sake of any specific woman at all, for example, one written as a generic document for people learning the craft, is invalid and is not a valid get.
"Keitzad? Hayah over bashuk v'shama kol sofrim makrin": - How so? A man was passing through the marketplace and heard the voice of scribes dictating.
A man passes through the marketplace and hears scribes dictating to their students, teaching them how to write a get, saying to them: "Ish peloni megaresh es pelonis mimakom peloni" - so-and-so divorces so-and-so from such-and-such a place. That passerby says: "Zeh shemi v'zeh shem ishti" - this is my name and this is my wife's name. It is invalid for him to divorce with it, since it was not written for the sake of divorce at all, but merely as practice.
From here on the mishnah uses the phrase "yeser mikan" - beyond this: even in cases that do contain some element of lishmah, the get is still invalid:
He wrote it to divorce his wife and then changed his mind: A man wrote the get for his own wife, and then changed his mind and decided not to use it, because they had made peace and were not divorcing. A fellow townsman found him - someone living with him in the same city, so that the name of the place in the get fits him as well - and said to him: "Shemi k'shimcha v'shem ishti k'shem ishtecha" - my name is like your name and my wife's name is like your wife's name, and therefore I will use the get that you no longer want. It is invalid for him to divorce with it, because although it was written for those same names, it was not written for that same person; someone else was in the writer's mind.
Two wives with identical names: "Hayu lo shtei nashim ushman shaveh, v'chasav l'garesh bo es hagedolah - lo yegaresh bo es haketanah" - if he had two wives whose names were the same, and he wrote a get to divorce the older one, he may not divorce the younger one with it. One who wrote a get in order to divorce the older wife cannot use that get to divorce the younger one in her place.
Leaving the decision until after the writing: A man who tells the scribe to write the get in such a way that he can divorce whichever of his wives he wishes, since their names are the same and he will decide afterward - this get too is invalid, since at the time the get is written it must be fixed and clear for whom it is being written.
In summary: The mishnah teaches four levels of invalidity in the requirement of lishmah: a get written as practice and not for the sake of any woman at all; a get written for the sake of one woman and then brought to serve a different couple bearing the same names; a get written for one woman and then brought to divorce her co-wife who has the same name; and a get written in doubt as to which of the wives it will serve. This is the requirement that a get be written lishmah - for the sake of divorcing this particular man from this particular woman.