Non est factum

(Latin) principle that can void a signed contract if certain types of errors were made (i.e. unintentional misunderstanding of the contract)

Non est factum –  Latin for "it is not [my] deed" – is a doctrine in  contract law that allows a signing party to escape performance of the agreement. 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 make the contract voidable.
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The plea of the general issue in an action of debt on bond.
  

Latin for not his deed and a special defense in contract law to allow a person to avoid having to respect a contract that she or he signed because of certain reasons such as a mistake as to the kind of contract. - (read more on Non est factum)