Daf Yomi Yevamot 115a/121a: The Law By A Single Witness of a Drowning in Water Which Has No End -- The Difference A Single Letter Makes

That single letter being either an aleph or a yud. Or perhaps a heh.

From my Rif blog (to be posted later), from Yevamot 115a (as well as from 121a):

איבעיא להו עד אחד במלחמה מהו

טעמא דעד אחד דמהימן משום דמילתא דעבידא לאיגלויי לא משקר ה"נ לא משקר
או דילמא טעמא דעד אחד משום דהיא גופה דייקא ומינסבא והכא זימנין דסניא ליה ולא דייקא ומינסבא

ואתינן למפשטה מהא דתניא מעשה בשני תלמידי חכמים שהיו באין עם אבא יוסי בן סימאי בספינה וטבעו והשיא רבי את נשותיהן ע"פ נשים והא מים כמלחמה דמו ומאה נשים כעד אחד דמו וקתני והשיא רבי את נשותיהן
אלמא דעד אחד נאמן במלחמה

It was a question to them: A single witness, by war, what? The reason that a single witness is believed is that facts which will eventually be revealed, he will not lie, and so too here, he will not lie. Or perhaps the reason of a single witness {being believed} is that she herself will delve into the matter before remarrying, but here, there are times that she hates him and will not delve into it before remarrying.
And we come to resolve it from this that they learnt {in a brayta}: There was an incident with two Torah scholars who came with Abba Yossi ben Simai in a boat, and it sank, and Rabbi married off their wives based on the testimony of women.
And behold, water is like war, and 100 women are reckoned as a single witness, and it teaches "and Rabbi married off their wives." Thus it is clear that a single witness is believed in war.

ודחינן ותסברא והא מים שאין להן סוף נינהו ומים שאין להם סוף אשתו אסורה
אלא היכי דמי הכא כגון דקאמרן אסוקינהו לקמן וחזונוהו לאלתר וקאמרי סימנין דידהו דלאו עלייהו קא סמכינן אלא אסימנין הוא דקא סמכינן וליכא למגמר מינה דסמכינן אנשים והוא הדין לעד אחד ולא אפשיט בעיין

And we reject it as follows: Now reckon! If it were water that had no end, in water which had no end his wife would be forbidden! Rather, how so here? Such as where they {=the women} say that it cast them before us and we saw them immediately,
{Yevamot 115b}
and they give their identifying signs -- such that it was not upon them that we rely, but rather we rely on the signs, such that there is not to learn from this that we rely upon women. And the same is the law for a single witness.
And so we do not resolve our question.

אלא מיהו כיון דאמרינן בפרקא דלקמן ההוא גברא דטבע בדיגלת ואסקוה אגישרא דשביסתנא ואסבא רבא לדביתהו אפומא דשושבינא לבתר חמשא יומא שמעינן דעד אחד במים שאין להם סוף נאמן
ודוקא היכא דאמר אסקוה לקמאי וחזיתיה לאלתר ואישתמודענא ליה דאיהו פלוני וכד אסיקנא למעשה דשני ת"ח שהיו באין עם אבא יוסי בן סימאי וכדאמרינן בההוא גברא דטבע בדיגלת ואסקוה אגישרא והוא הדין לעד אחד במלחמה היכא דאמר מת וקברתיו ואי לא קא מסהדי

Rather, however, since we say in the coming perek {121a} that "a certain man drowned in Diglat {=the Tigris river} and they took him out on the Shebistana bridge, and Rava permitted his {=the man's} wife to marry, on the say-so of the groomsman, after five days," we deduce that a single witness is believed even by water which has no end.
Etc.

Thus, we see that we acted upon the say-so of a single witness, the שושבינא, the groomsman. The kametz aleph ending is the definite article, though in Babylonian Aramaic you can have that kametz aleph ending even for the absolute form (without "the").

However, in our girsa in our gemara, we have שושביני, which is the plural. If so, Rava is not relying on a single groomsman, but rather upon multiple groomsmen. (And indeed, if you check the Soncino translation, you will see "and, on the evidence of the shoshbinim," and in a footnote, he writes "Pl. of shoshbin, groomsman'."

If so, the Rif's proof from the single witness is no proof.

Looking at the commentary of the Ran, on the side of the Rif, you see a third option -- שושביניה, which is the same as our gemara but with a (mapik) heh at the end. This is the possessive -- his bridegroom, which is once again singular.

This is why girsology is so critical. On the basis of a single letter difference, you can derive a major halachic principle, either correctly or incorrectly.

Update: Just how does such a girsological difference arise? The most obvious way is that a scribe wrote שושבינ'גת, with an apostrophe after the nun. Another sofer then expanded it in the wrong direction.

Update: Later in Rif, when he actually reaches the gemara, he has the same girsa as the Ran - שושביניה, with a yud heh rather than with an aleph. So perhaps earlier he was paraphrasing. Regardless, the issue is still the presence or absence of heh, which will give us plural or singular.