Non est factum - "it is not my deed". A plea that a person who has signed up to a deed or a contract lacked the necessary understanding or intent, and is therefore not bound by the document. A successful plea would void the contract.

A claim of non est factum means that the signature on the contract was signed by mistake, without knowledge of its meaning, but was not done so negligently. A successful plea would void the contract.

A person who signs away the deed to a house, thinking that the document signed was only a guarantee for another person's debt, might be able to plead non est factum in a court and on that basis get the court to void the contract.

A refund claim filed before authority not having territorial jurisdiction is not ab initio void or non est factum – CEGAT

Mistake as a ground for relief. At common law mistake was admitted as a foundation of relief in three cases only, namely (1) in actions for money had and received to recover money paid under a mistake of fact; (2) in actions of deceit to recover damages in respect of a mistake induced by fraudulent misrepresentation; and (3) as a defence in actions of contract where the mistake of fact was of such a nature as to preclude the formation of any contract in law, for example, where there was a mutual mistake as to the subject matter of the contract, and, therefore, no consensus ad idem by the parties, or where the mistake was made as to the identity of one of the parties and such identity was an inducement to the other to enter into the contract, or where the mistake related to the nature of the contract under such circumstances as would, if the contract were embodied in a deed, justify a plea of non est factum.- Halsubry’s Laws of England,