Nedarim 26 - Clear mistakes
One can make an obvious mistake in his vow, and if so, the vow does not count, and he does not have to fulfill it. For example, "This food is prohibited to me by a vow if I ate anything today - or if I drank today" - and then he remembers that he did eat or drink.
Another example: "Let my wife be prohibited to me by a vow since she stole my wallet - or since she hit my son" - and it turns out that she did not do such thing. Yet another example: he sees people eating his dates, and vows the dates to be prohibited to them, and then discovers that his father or his brother were with these people - all such vows are void.
In the same way, mistaken oaths are void. For example, Rav Kahana and Rav Asi both took an oath that their teacher said something, and their oaths clearly contradicted. When it was found out that one of them was actually correct and supported by others, the oath was still not a false oath, because as far as the other one could ascertain, he believed to be saying the truth.
Art: Peasants preparing food in a courtyard by North-Italian School