A defence of justus error raises a triable issue where a signatory alleges that a suretyship clause was surreptitiously hidden in a contract, was not brought to her attention by the offeror, and she would not have signed had she been aware of it — particularly where there was a prior interaction with the offeror's representative giving rise to an assumption that no such clause existed. In such cases the dispute of fact should be ventilated at trial with oral evidence. Additionally, where a defendant relies on the Conventional Penalties Act to seek reduction of a penalty, and the information needed to quantify the reduction lies exclusively within the knowledge of the plaintiff, the defendant need not specify the exact quantum of reduction at the summary judgment stage — it is sufficient to set out the grounds for reduction with sufficient particularity to disclose a triable issue.