[2] According to Saunders v Anglia Building Society [1971] AC 1004,[3] applied in Petelin v Cullen [1975],[2] the strict requirements necessary for a successful plea are generally that: Non est factum is difficult to claim as it does not allow for negligence on the part of the signatory; i.e. failure to read a contract before signing it, or carelessness,[2]: para 12 will not allow for non est factum.
The High Court of Australia found that because of Petelin's mistaken belief which was not because of his carelessness, his claim of non est factum was successful.
[2]: para 15 [4] In the English case of Lloyds Bank v Waterhouse[5] a father acted as a guarantor to his son's debt when purchasing a farm.
The father was illiterate and signed the bank document under the belief that he was acting as the guarantor for the farm only, when the contract was actually for all the debt accumulated by the son.
[9] The Court dismissed the argument that the appellant had been careless as that would presume that he was capable of turning his mind to the issue and making judgements.