1) 27b: Mishna
Why did R' Zecharya's forced to divorce his wife, after all he has a "Taanat Bari Li" the same claim that allows a wife to marry one of the witnesses that saw her husband dead?
2) 28a: Mangedinan Lehu
Ena Nifraat Ela Al-yede Acher - if she sues her ex-husband she must do it with someone else. Rav Sheshet says that if they come to beit din we will punish them .
Why do we punish him, if she sues him, he is forced to come to beit din. We'd rather just punish her.
1) Bari Li only helps in a case of Trei u'Trei, where Beis Din has a Safek. However, when there is no contradictory Eidus, Beis din is obliged to follow the Din even against the Bari.
2) The Lashon of the Gemara is "I Asu l'Kaman l'Dina" which means they came together willingly. It does not relate to the above case of Nifra'as.
Dov Zupnik