Nedarim 87A

והא גבי קרעים דכתיב ״על״ ״על״, דכתיב: ״על שאול ועל יהונתן בנו״.

The Talmud comments: But is it not so that with regard to the tears in one’s clothing that are made for the dead, as it is written “for,” “for,” and about which is written: “And David took hold of his garments and rent them, and likewise all the men that were with him, and they wailed, and wept, and fasted until the evening, for Saul, and for Yonatan his son, and for the people of YHWH, and for the house of Israel, because they were fallen by the sword” (II Samuel 1:11–12). The use of the word “for” with regard to each of them indicates that one must make a separate tear in his garment for each person who died.

ותניא: אמרו לו ״מת אביו״ וקרע, ואחר כך נמצא בנו ⁠— יצא ידי קריעה.

The Talmud asks: And yet it is taught in a baraita: If they said to him that his father had died and he rent his garment over his death, and afterward it was discovered that it was not his father who died, but his son, he has fulfilled his obligation of rending his garment. This shows that even if a person mistakenly tore his garment for the wrong person he has nevertheless fulfilled the obligation. Here too, if a man nullified the vow of his wife, thinking that it was the vow of his daughter, his nullification should be effective.

אמרי, לא קשיא: הא בסתם, והא במפרש.

The Talmud responds: The apparent contradiction is not difficult. That baraita refers to a case where he received a non-specific report, i.e., he was told that an unspecified relative died. In such a case his obligation to rend his garment has been discharged. And this Mishnah refers to a case where the bearer of the news mistakenly specified that his daughter had taken the vow, when in reality his wife had. In such a case, his nullification is ineffective.