What is Ambiguity Upon The Factum?

Legal Definition
An ambiguity in relation to the very foundation of the instrument itself, as distinguished from an ambiguity in regard to the construction of its terms. The term is applied, for instance, to a doubt as to whether a testator meant a particular clause to be a part of the will, or whether it was introduced with his knowledge, or whether a codicil was meant to republish a former will, or whether the residuary clause was accidentally omitted. Eatherly v. Eatherly, i Cold. (Tenn.) 461, 465, 78 Am. Den. 499.
-- Black's Law Dictionary