Yevamot 84B

וכי תימא: קסבר האי תנא נישואין הראשונים מפילין ⁠— הרי חלל שנשא כשרה, ולא אמרינן נישואין הראשונים מפילין!

And if you would say that this tanna holds that it is her status at the time of the first marriage that determines her status when she happens before the yavam for levirate marriage, and not her status at the time of her husband’s death, and therefore if she had been a virgin when she married she would have been permitted to enter into levirate marriage with a High Priest, that cannot be so, as the Mishnah cites the case of a ḥalal who married a woman fit to marry a priest, in which case the woman is permitted to her husband and forbidden to her yavam. And in that case, we do not say that the first marriage determines her status when she happens before him for levirate marriage, as the woman was fit to marry the brother when she married the ḥalal and was rendered a ḥalala only as a result of her marriage.

הא ודאי משום סיפא, משום דקבעי למיתנא סיפא: כהן גדול שנשא את האלמנה, ויש לו אח כהן גדול או כהן הדיוט. דוקא אלמנה ⁠— אבל בתולה חזיא ליה, משום הכי קתני אלמנה.

The Talmud answers: That fact, that the Mishnah cited a case where the priest married a widow, is certainly due to the latter clause of the Mishnah, i.e., due to the fact that the tanna wants to teach in the latter clause: A High Priest who married a widow, and he has a brother who is the High Priest or a common priest. In the case where the yavam is a common priest, the widow is forbidden to him specifically if the High Priest married a widow, as by consummating the marriage he rendered her a ḥalala. However, had she been a virgin when the High Priest married her, he would not render her a ḥalala, and when the High Priest dies she would be fit for his brother. It is due to that reason that the tanna teaches the case of a widow in the first clause as well.