Gittin, Chapter 4, Mishnah 1. This mishnah addresses the following question: if a man sent an agent to deliver a get to his wife, may he change his mind and annul the get?
The text of the mishnah:
"HaSholeiach get l'ishto" - the husband sends a "shaliach l'holachah", an agent who represents the husband and not the wife. In what manner did he cancel the agency?
"Higia b'shaliach" - after sending him, the husband came upon the agent again and stopped him.
"O sheshalach achar shaliach" - or he sent a second agent after him.
In both cases - "v'amar lo: get shenasati lecha batel hu", whether he said this himself or the second agent whom he appointed said it - "harei zeh batel", and the get is null and void.
Two novel points in this ruling:
Where he happened upon the agent: we do not say that since he did not actively chase after the agent but merely came upon him by chance, his words are nothing more than an attempt to make life difficult for his wife. Rather, we say that he genuinely meant what he said, and the annulment takes effect.
Regarding the second agent: we do not say that the second agent lacks the power to cancel the agency of the first agent. He does have that power, since he was sent by the husband.
Annulment in the presence of the wife:
"Kadam etzel ishto" - the husband reached his wife before the agent handed her the get, "o sheshalach shaliach" - or he sent her an agent who reached her before the get arrived, "v'amar lah: get sheshalachti lach batel hu" - "harei zeh batel". Here too we do not say that his whole intent is merely to make things difficult for her; rather, we take his words seriously and the get is void.
Once the get has reached her hand:
In all of these cases, "mishehigia get l'yadah - shuv eino yachol l'vatlo". Even if the husband is seen struggling and doing his utmost to reach her and tell her that the get is annulled, since he had no opportunity to do so before the get reached her hand - it is too late. Once the get has reached her hand he no longer has the power to annul it, and she is divorced.
In summary: we have learned that as long as the get has not reached the wife's hand, the husband may annul it - whether by himself or through an additional agent, and whether before the agent of delivery or before his wife; but once the get has reached her hand, he no longer has the power to annul it and she is divorced.