Agunah

 https://en.wikipedia.org/wiki/Agunah

Finding a flaw in the marriage ceremony is considered to be a last resort in releasing an agunah. It is rarely used as it is typically difficult to find actual cause in most marriages sufficient to invalidate them retroactively. In Jewish law, a marriage must be performed in front of two witnesses. In order to release the agunah, efforts are made to identify reasons why one of the witnesses was ineligible. This is typically unachievable as strong efforts are made at the time of marriage to ensure the validity of the witnesses and the marriage ceremony. Another possibility is to prove that the woman did not consent to the marriage clearly and of her own free will, so that the marriage ceremony is declared invalid. This too is not generally accepted amongst the halakhic authorities as there is generally no method to disprove intent. It is felt that the purpose of this endeavor is solely or primarily to retroactively delegitimize a marriage that was performed and accepted often many years previously. Annulling the marriage has no impact on the status of the woman's children. However, since it is not a generally accepted mechanism, it may leave the wife susceptible to a halakhic ruling that she was still married, and any subsequent relations with another man to be adultery. And it may lead to other halakhic problems, so it is only used as a last resort by the authorities that do accept its use.