Unilateral mistake under the traditional rule is not grounds for rescission unless ...
(1) the non-mistaken party knew or should have known of the mistake,
(2) the non-mistaken party caused the mistake, or
(3) the non-mistaken party had a duty to disclose or failed to disclose the mistake.
But, under the modern rule, unilateral mistake is grounds for rescission if ...
(1) the party seeking rescission is not at fault,
(2) the mistake is vital or essential,
(3) enforcement would be unconscionable,
(4) prompt notice is given of the election to rescind,
(5) the mistake is one of calculation, not judgement, and
(6) the status quo ante maybe be restored.