Nedarim 27B

דאי הוה ידע דמית, מן לאלתר הוה גמר ויהיב גיטא.

for had he known that he would die within a year he would have immediately finalized his decision and given her the bill of divorce. Since he gave it to her initially so that she not require levirate marriage, it is assumed that his intent was to deliver it even in this case. By contrast, in the case where one stipulated about his rights, which he certainly did not intend to forfeit, it is assumed that he would not have wanted his statement to take effect in this situation.

מאי שנא מההוא דאמר להו: אי לא אתינא מכאן עד תלתין יומין ליהוי גיטא. אתא ופסקיה מעברא. אמר להו: חזו דאתאי, חזו דאתאי! ואמר שמואל: לא שמיה מתייא. אמאי? והא מינס אניס!

The Talmud continues to question Rava: In what way is it different from the following case: There was a certain man who said to the agents with whom he entrusted a bill of divorce: If I do not return from now until 30 days have passed, let this be a bill of divorce. He came on the 30th day but was prevented from crossing the river by the ferry that was located on the other side of the river, so he did not arrive within the designated time. He said to the people across the river: See that I have arrived, see that I have arrived. And Shmuel said: It is not considered to be an arrival, and the condition is considered to have been fulfilled. The Talmud asks: Why is it not considered an arrival; but he was impeded by circumstances beyond his control?

דלמא אונסא דמיגליא שאני, ומעברא מיגלי אונסיה.

The Talmud responds: Perhaps the case of circumstances beyond one’s control that are apparent to everyone and could have been anticipated ahead of time is different, and a ferry is considered an apparent type of circumstance beyond one’s control, which he should have considered and stipulated explicitly. Since he did not do so, it is not considered a circumstance beyond one’s control.