פירוש על יבמות 10:2

A woman who was married by mistake and it is known that she is forbidden to her husband, and the beis din sends her out from him [makes them divorce]: if she is a minor [under 12] and is not able to give birth, she does not need to wait [before marrying someone else], because this is not common and anything that is uncommon within a majority, they [the Rabbis] did not decree upon it.

Two witnesses say: He died, and two witnesses say: He did not die, she may not be married and if she is married she must be divorced, for the matter is doubtful. And if she married one of her witnesses, and she says: I am certain that he has died, she need not be divorced. We don't force a wife to leave her husband if a rumor goes out that her first husband is alive (Rabbenu Yeruham).

If she did not marry a second man but was merely engaged and then her husband returns, she does not need a Get from the second man and she can return to her husband. She would be permitted to the second man in the event her husband dies or divorces her [Rema: A woman who was engaged and mistakenly thought she was not engaged and married another man, she must leave both men and all laws stated above apply to her. But if they forced her to get married or if the courts ruled in error and she married on their word, it is as if she was raped and she is still permitted to her first husband (The Rashbah's commentary brought by the Beit Yosef)].