Gemara

2b (Beraisa #1 - R. Shimon): If Kemitzah of any Minchah was Lo Lishmah, it is Kosher and atoned. Menachos are unlike Zevachim. If Kemitzah was l'Shem Marcheshes and it was really (cooked in a) Machavas, this is evident.

Zevachim are different. Shechitah and Zerikah are the same for all of them.

(Rabah): R. Shimon holds that intent for a different owner is not evident, so it is Posel. Intent for a different Korban is evident, therefore the intent is Batel.

3b (Rava): The Beraisa says that R. Shimon is Machshir (l'Shem) Menachos because the Lo Lishmah is evident. It means that even though the intent is not evident (that it is Lishmah. Just the contrary, the Lo Lishmah is evident!) they are fully Kosher. (Rava holds that if the Lo Lishmah is evident, this is reason to be Posel, the opposite of Rabah's reasoning.)

Gitin 4a: R. Elazar holds that a Get without signatures is Kosher, but if it has signatures, they must be Lishmah;

(R. Aba): R. Elazar agrees that a Get that is Mezuyaf mi'Tocho (the signatures do not truly testify to what is written) is Pasul.

86a (Mishnah): The following Gitin are Pasul, but if she remarried, the children are Kosher (not Mamzerim):

It is written in Kesav Yado (his, i.e. the husband's, own handwriting), but witnesses did not sign it;

Only one witness signed it.

R. Elazar says, even if no witnesses signed on it, but it was given in front of witnesses, it is valid.

Sanhedrin 28b: Two men whose wives were sisters signed on a gift document. Rav Yosef suggested that the giver give the document to the recipient with Edei Mesirah (witnesses who saw it given, for they are Karsei, i.e. empower a document), according to R. Elazar.

Objection: R. Aba taught that R. Elazar admits that a document that is Mezuyaf mi'Tocho is invalid!

Bava Basra 162a (Beraisa): If the witnesses left two blank lines between the text and their signatures, it is invalid.

We are concerned lest the bearer add clauses to his benefit. We are not concerned when one blank line was left, for we do not learn from the last line.

(Chizkiyah): If blank lines between the text and the signatures were filled with signatures of relatives, it is valid.

Rishonim

Rif (Gitin 47b): Rav holds that when one witness signed, the (Get is Batel and) the child is a Mamzer. He rules like R. Elazar, that Edei Mesirah Karsei, i.e. a Get is Kosher without any signatures. Why is one witness worse than none?! Rather, a Get with one signature is Batel when there are no Edei Mesirah. If it had two signatures, it would be Kosher without Edei Mesirah. A Gaon wrote that R. Elazar says that a Get with one witness and in the scribe's handwriting is Batel, for it is Mezuyaf mi'Tocho. This is unlike I wrote.

Ran (DH v'Hainu): R. Elazar does not require signatures, but when witnesses sign, they miust sign Lishmah. If not, the Get is Mezuyaf mi'Tocho. If only Edei Mesirah Karsei, and signatures merely indicate that he gave it to her, why do we care if they signed Lishmah? Rather, R. Elazar holds that when there are Edei Chasimah, they are Karsei. When two Gitin are on one parchment, and the tops are near the other and witnesses signed in the middle, both are Pasul. The Gemara connotes that according to R. Elazar, if it was given in front of Edei Mesirah, they are Kosher. The Rambam rules like this. Since the Pesul is evident, is not Mezuyaf mi'Tocho, just like R. Elazar is Machshir a Get in the husband's handwriting or with only one witness if it was given in front of Edei Mesirah.

Rambam (Hilchos Gerushin 1:17): If even one of the Edei Chasimah was Pasul, and it was given in front of Kosher witnesses, it is Pasul, for it is Mezuyaf mi'Tocho.

Magid Mishneh: When there are signatures and they signed Lo Lishmah, or one of them is Pasul, this is worse than when there are no Edei Chasimah. It seems that the Rif disagrees. He learns from Rav that it is no worse when there is one witness. He argues with a Gaon; the Rif holds that the Get is only Pasul. Perhaps the Rif and Rambam agree; one witness and a scribe's handwriting is Kosher; unlike a Pasul witness and a Kosher witness. The Gaon calls both of these Mezuyaf mi'Tocho.

Rambam (18): If the witnesses distanced their signatures two lines from the Get, it is Pasul, if she has the Get and there are no Edei Mesirah. If it was given in front of witnesses, it is Kosher even if the witnesses signed far from the Get or no witnesses signed at all, for divorce is primarily through Edei Mesirah.

Ra'avad: I am stringent about this, lest he put a condition in those two lines, and she will show her signed Get, and it is Mezuyaf mi'Tocho, for people will come rely on the signatures.

Magid Mishneh: If there are Edei Mesirah there, it is not Mezuyaf mi'Tocho, for the witnesses are not read with it. A Tosefta (Gitin 7:9/1says that if the witnesses were two lines from the writing, it is Pasul.

Lechem Mishneh: The Rambam holds that this is unlike a Get written Lo Lishmah, or with only one Kosher witness. Those are Mezuyaf mi'Tocho. Here, it can be forged, but now it is not forged. This is like erased paper. It is Kosher, even though one could (erase it again and) add things, since Edei Mesirah Karsei. The Ra'avad disagrees, for here one can add things and people will rely on it, for the forgery will not be known. People know that one can forge on erasable paper. His reasoning is better.

Rambam (Hilchos Malveh 27:4): If witness signed two lines from the end of the document, it is Pasul. If the space in between was filled with Pasul witnesses or relatives, it is Kosher, for one cannot forge it. If it was filled with scratches of ink, it is Pasul, for perhaps they signed on the scratches, and not on the document itself.

Rosh (Bava Basra 10:2): If the witnesses distanced two lines (from the end of the document), even if it says Sharir v'Kayam (these words denote the end of a document) at the end, it is Pasul. We are concerned lest one cut off the document and write a document in one line, and recap in it the line below. Even if it is known that nothing was added, even if the space in between was Metuyat (filled with dots of ink), it is Pasul, lest people say that they signed on the dots, and not on the document itself. The Rashbam says that when they left two blank lines it is Pasul even if witnesses say that nothing was added. The document is worthless, because it was not done like Chachamim enacted. The entire Sugya proves that any document that can come to forgery, even if it was not forged yet, is Pasul, for it is unlike Chachamim's enactment.

Poskim

Shulchan Aruch (CM 45:6): Witness must be careful not to distance two lines from the end of the document. If they did, it is Pasul, even if it says Sharir v'Kayam, even if it is known that nothing was added, even if the space in between was Metuyat, it is Pasul, lest people say that they signed on the Tiyut, and not on the document itself.

Shach (12): The Beis Yosef says that we are concerned lest he cut off the top and write another document above the signatures. Even without this, perhaps he will erase Sharir v'Kayam, write a Kiyum (acknowledgement of the erasure in the document), and add new matters in the blank lines.

Gra (14): The Gemara says that if the blank lines were filled with Pasul witnesses, it is Kosher. This implies that before filling it, it is Pasul.'

Shulchan Aruch (14): If the witnesses signed two lines below the document, and the blank lines were filled with Pasul witnesses or relatives, it is Kosher, for it cannot be forged. If the document was seen in Beis Din before the blank lines were filled, it cannot be made Kosher.

Shulchan Aruch (EH 130:1): Two Kosher witnesses must sign the Get (Rema - one under the other) below the bottom line. They should not leave space for two lines between the bottom line and their signatures.

Rema: If they left blank space, the Get is Pasul.

Beis Yosef (DH Kasuv bi'Teshuvas): The Rashbatz (1:7) says that if a witness signed the wrong name, and then his real name, the Get is Pasul, for his first signature cannot be validated and it preceded his true signature. It is Mezuyaf mi'Tocho.

Beis Shmuel (1): This is unlike filling blank lines with Pasul witnesses, which is Kosher (CM 45:14) On a Get, we do not say that they signed Pasul witnesses to fill the blank lines. The Taz equates these two laws. I disagree.

Beis Yosef (DH veha'Rambam): The Rambam holds that any Get that is recognizably Pasul, if it was give in front of Edei Mesirah, it is Kosher. The Ramban and Rashba agree. The Rambam would say the same when the witnesses signed two lines below the Get; he mentioned only the cases in the Mishnah.

Beis Shmuel (2): Even though a Get written on a surface that can be (erased, and thereby) forged is Kosher through Edei Mesirah (124:2), there is different, for it is evident that it can be forged, so we do not rely on the Edei Chasimah. Here, perhaps he will fill the blank lines, and no problem will be evident.

Rema (ibid.): Some disagree and are Machshir through Edei Mesirah.

Beis Shmuel (3): The Rambam is Machshir. In CM 45:6 (regarding monetary documents), the Mechaber did not distinguish between whether or not there are Edei Mesirah here. This is even more difficult for the Rambam, who holds that Edei Chasimah alone suffice for Gitin, yet he rules that it is Kosher with Edei Mesirah, but in Hilchos Malveh he disqualifies! Perhaps monetary documents are different, for they must be able to endure for a long time.

Gra (5): The latter opinion is Machshir through Edei Mesirah, just like we are Machshir on a surface that can be forged (124:2). This is unlike Mezuyaf mi'Tocho. There, there is a Pesul now (Lechem Mishneh). The Rambam distinguishes, for people will not rely on the signature if the surface can be forged. Here, if they will fill the empty lines, the forgery will not be evident.

Rema: If the last witness signed first, and then the first witness signed, it is Kosher, even though initially the second signed two lines from the Get.

Gra (6): If there were two blank lines above the signatures, and they were filled with Pasul witnesses, it is Kosher, and all the more so here.