The principal term of contract A was the irrevocability of the bid and the corollary term was the obligation in both parties to enter into a construction contract, contract B, upon the acceptance of the tender. The deposit was required to ensure the performance by the contractor-tenderer of its obligations under contract A. It is not correct to say that when a mistake was proven after the tenders were opened by the production of reasonable evidence, the person to whom the tender was made could neither accept the tender nor forfeit the deposit. The test was to be imposed when the tender was submitted, not at a later date, and at that time the rights of the parties under contract A crystallized, at least in circumstances where the tender was capable of acceptance in law.