The Mishna on our daf deals with the case of nidrei onasin, which, as we learned (on daf 21) do not need to be annulled. The example suggested by the Mishna is a case where a person makes a neder obligating someone to come and join him in a meal, but then the invited guest or one of his children becomes ill or cannot cross the river to attend the meal.
Rava explains that this ruling is based on a general principle of the Talmud called anoos rahamana petarei – someone who cannot perform an obligation for reasons that are beyond his control is exempted by God. Our Gemara points out, however, that there are other cases of onasin in which we do not apply the rule of anoos rahamana petarei. An example is the case of a conditional divorce in which a man writes a geṭ to his wife that says, “This will be your bill of divorce if I do not return within 30 days” and at the end of the 30 days the husband is stranded on the wrong side of the river with no ferry to take him across. Even though he is shouting, “I have arrived! I have arrived!” we do not consider him to have come back and the divorce goes into effect.
The Gemara explains that being stranded on the wrong side of the river with no available ferry to take him across was an ones that should have been anticipated and made part of the conditions of the divorce. Since it was not done that way, we do not consider it to be a true ones.
The ferry to which the Gemara refers here is called a ma’abera – a small boat or raft that took people across the river. Such ferries usually made several crossings every day, with a group of people each time. Generally speaking, it was not worthwhile for the owner of the ferry to cross the river with just one passenger, and people needed to wait until the boat filled up before it would set sail. When the rivers were wide and deep – as was the case in Babylon – there was no other way to cross the river aside from these ferries, and if the boat was on the opposite bank of the river it may have even been possible that the boat’s captain would not realize that someone was waiting on the other side of the river for him. Thus, problems of crossing the river should have been taken into account by someone who made a condition to return by a certain time.