Yevamot 88 - Her husband died
If a woman's husband went overseas, and subsequently a man came back and told her that her husband died, and that he witnessed this, and he is willing to testify in court, then ordinarily this is not sufficient for her to remarry, because the rule is that there must be two witnesses to establish any event.
However, since often there won't be two witnesses, the Sages have established a new rule: that one witness is sufficient in this situation, and that after the court examines him and issues a ruling permitting her to remarry, she can do so.
This is based on the assumption that the woman does not take the matter lightly, but will rather investigate on her own. And yet, in abrogating the law of two witnesses, she is taking the matter into her own hands. If it later happens that the husband was really alive, and he comes back, then she - due to her lack of sufficient investigation - will face a penalty: she will have to leave her new husband (to whom she was never legally married, as it turns out, because she was still married to the first one), and she will also have to be divorced from the first husband, all as a deterrent.
Art: Dead men tell no tales by Howard Pyle