Two litigants were disputing
ownership of a property. One said, “It was the land of my forefathers,”
and produced witnesses to that effect. The other one produced witnesses
that he used the land for three years, needed to establish chazakah.
Rabbah said, “Why should he lie?” The current occupant could have kept
the land claiming that he bought it, so we should believe him when he
says that he inherited it. But Abaye said that “Why should he lie?”
can't be used to contradict witnesses.
The current occupant then changed his claim, saying “Yes, the land
indeed belonged to your forefathers, but I bought it from them, and I
feel as secure as if it were my forefathers'” and this claim was
accepted.
The general rule is: one cannot enter a claim that completely
contradicts his previous claim, but he can add information or amend the
previous claim by reinterpreting it.