Mistake as to the promise known to the other party

Again these cases could be looked at as "formation of the contract" cases: can it be said that in these cases the parties are ad idem?

General rule:

Although the conduct of one party is such that a reasonable man would assume that he was assenting to the offer of the other party, yet if that other party knows that he does not really assent there will be no genuine agreement.

However, the mistake must relate to the promise - the actual offer - any other mistake eg as to the quality or nature of the thing contracted for will not act as an operative mistake. This rule is known as the rule in Smith v Hughes.