The court described the mistake as a 'retrospective mistake by means of fresh evidence', noting that the source documents relating to the unclaimed portion of expenses constituted evidence that came to light only after the court had considered the vouchers and given judgment. The court emphasized that attempting to characterize a unilateral mistake as a mutual mistaken assumption is merely an attempt to 'clothe a unilateral mistake in another garb' and cannot succeed. The judgment reinforced general principles regarding when parties can escape contracts due to fault in their own mistakes, citing Christie's extensive examples including: not carrying out reasonably necessary investigations, not reading the contract before signing, carelessly misreading terms, not having the contract explained in an understandable language, misinterpreting clear and unambiguous terms, and any circumstances where the mistake is due to carelessness or inattention.