Mishnayos Yevamos Perek 3 Mishnah 8

And if any of these fifteen women who are prohibited as forbidden relatives had undergone a betrothal or divorce whose status is uncertain with the deceased brother, then those women who were their rival wives must perform ḥalitza and may not enter into levirate marriage since they are possibly the rival wives of forbidden relatives. The mishna elaborates: How could there be a situation of uncertainty with regard to betrothal? If in the public domain he threw her an item for the purpose of betrothal and there were eight cubits between them, and the item was possibly closer to him and did not enter into her domain, and possibly closer to her, i.e., within four cubits of her, whereby she could acquire the object, this is a case of uncertainty with regard to betrothal. Uncertainty with regard to divorce occurs when, for instance, he wrote a bill of divorce in his handwriting but there are no signatures of witnesses on the document, or there are the signatures of witnesses on the document but there is no date written in it, or the date is written in it but there is only the signature of a single witness. Since there is doubt as to whether these three kinds of bills of divorce are valid, a woman who was divorced through them is only possibly divorced, and so this case is called uncertainty with regard to divorce.

וְכֻלָּן שֶׁהָיוּ בָהֶן קִדּוּשִׁין אוֹ גֵרוּשִׁין בְּסָפֵק, הֲרֵי אֵלּוּ צָרוֹת, חוֹלְצוֹת וְלֹא מִתְיַבְּמוֹת. כֵּיצַד סְפֵק קִדּוּשִׁין, זָרַק לָהּ קִדּוּשִׁין, סָפֵק קָרוֹב לוֹ סָפֵק קָרוֹב לָהּ, זֶהוּ סְפֵק קִדּוּשִׁין. סְפֵק גֵּרוּשִׁין, כָּתַב בִּכְתַב יָדוֹ וְאֵין עָלָיו עֵדִים, יֵשׁ עָלָיו עֵדִים וְאֵין בּוֹ זְמָן, יֶשׁ בּוֹ זְמָן וְאֵין בּוֹ אֶלָּא עֵד אֶחָד, זֶהוּ סְפֵק גֵּרוּשִׁין: