1) THE CONCERN OF "SHEMA PIYES"
QUESTIONS: The Gemara introduces a concern which should prohibit a Shali'ach from delivering a Get. This concern is "Shema Piyes" -- perhaps the husband appeased his wife (and no longer wants to divorce her). The Gemara states that because of this concern, when a man instructs a Shali'ach to deliver a Get to his wife after thirty days have passed, or to deliver a Get which includes a stipulation that it should not take effect until after twelve months have passed, the Get may not be used unless it is known for certain that the husband did not appease his wife. One way to know for certain that the husband did not appease his wife is by having the husband stipulate that he trusts his wife to testify whether or not he appeased her. Another way to know for certain that the husband did not appease his wife is when his wife is only an Arusah (and thus they were not together).
RASHI explains that the problem with the Get in a case where the husband might have appeased the wife is that he might have had relations with her before the time at which the Get took effect, which renders the Get a "Get Yashan." The Gemara earlier (25b) explains that the reason why the Chachamim invalidated a Get Yashan is that the date on the Get might precede the date of conception of a child born during the marriage, and this people will assume that the child was born out of wedlock.
TOSFOS (18b, DH Shema Piyes) questions Rashi's explanation.
(a) According to Rashi, who explains that when the husband appeases the wife there is a concern that the Get might be a Get Yashan and the date of the Get will the date of the child's conception, why is the Shali'ach permitted to deliver the Get when the husband announces that he trusts his wife's testimony about whether or not he appeased her? Even if he trusts her, Beis Din cannot trust her, and therefore Beis Din should be concerned for the welfare of a child who was conceived after the date written in the Get, despite the woman's testimony.
(b) If the problem of "Shema Piyes" is that the husband might have lived with his wife before the Get took effect, how can this concern be applied to a case in which the husband gives a Get stipulating that it should take effect in twelve months if he does not return before the twelve months pass? If the husband was together with his wife, the Gemara should be concerned not that the Get is a Get Yashan, but that it is annulled altogether, since the condition which the husband stipulated (that the Get will take effect only if he does not return within twelve months) was not fulfilled (since he returned within twelve months)!
(a) TOSFOS there asks another question on Rashi from a Beraisa later (76b) which teaches that in a case of a Get which takes effect after thirty days, where the husband was not together with his wife during that time, there is no concern that the Get is a Get Yashan. The Gemara asks that there still should be a concern that "Shema Piyes," that he appeased her.
The Beraisa clearly implies that although there is no reason to be concern that the Get is a Get Yashan, there is a concern that the husband appeased his wife! It is clear from the Gemara there that the concern for Get Yashan and the concern for "Shema Piyes" are two completely different matters, in contrast to Rashi's explanation here.
Rashi there (DH Shema Piyes) answers this question by explaining that Beis Din certainly has no reason to be concerned that the husband clandestinely secluded himself with his wife and the date in the get predates the conception of their child. However, Beis Din should be concerned that the husband -- in an attempt to frustrate his wife -- will come at a later date and try to invalidate the Get by claiming that he secluded himself with her and that the Get is a Get Yashan. Just as the Chachamim took precautions to prevent the husband from invalidating the Get by saying that the Get he sent from Medinas ha'Yam is forged, they were also concerned that the husband should not be able to say that the Get is a Get Yashan. Hence, it is clear why there is need to be concerned when the husband announces that he trusts his wife's testimony: since he gives her the power to override whatever he says, there is no reason to fear that he will claim that the Get is a Get Yashan.
Tosfos himself (18b) in the name of the RI suggests that this is Rashi's intention.
(Although one opinion in the Gemara (79b) states that if the husband already divorced his wife with a Get Yashan she is permitted to marry someone else l'Chatchilah on the basis of that Get, Rashi there (DH Im Nisgarshah) explains that she may get married on the basis of the Get Yashan only when her husband has left to Medinas ha'Yam and she cannot get another Get from him. The concern in the case of the Gemara here is that the husband might claim that the Get is a Get Yashan and prevent her from marrying as long as he is in town. See also TOSFOS to Yevamos 14b, DH b'Get Yashan.)
(b) Apparently, Rashi understands that when the husband stipulates that the Get will take effect only "if I do not return within twelve months," he means that the Get will take effect only if he does not return permanently within twelve months. Coming back to visit during that time is not a contradiction to his condition and does not annul the ability for the Get to take effect after twelve months. Accordingly, the Get could take effect after twelve months if not for the concern of Get Yashan.
(RABEINU TAM, cited by Tosfos (18b), suggests another approach to the Sugya of "Shema Piyes" because of these questions.)