Estoppel is a common law doctrine which, when it applies, prevents a litigant from denying the truth of what was said or done.
Under the doctrine, the grantor of a deed (generally the seller of a piece of real property) is estopped (barred) from denying the truth of the deed.
If O conveys property she doesn't own to A by warranty deed, but O later acquires title to that land, then title immediately passes to A.
However, if, as above, O conveys property she doesn't own to A by warranty deed, but O later acquires title to that land, A may elect to treat O's lack of title at the time of the conveyance as a breach of the covenants of seisin and right to convey (two of the six traditional forms of Covenants for Title that are contained in a general warranty deed), and sue O for damages.
A cannot be forced to accept O's after-acquired title if she wishes instead to receive damages.