A contract can be voided under common law rules for mistake in the following situations:
Common mistake (where the mistake is shared by both parties, is fundamental and directly affects the basic definition of what the parties are contracting for). The mistake will render the contract void if it robs it of all substance.
Mutual mistake (where the parties are at cross-purposes with one another). If, from the parties’ words and conduct, only one possible interpretation of what was agreed can be deduced, the contract will still be valid. Otherwise it will be void.
Unilateral mistake (where one party is mistaken and the other knows or ought to have known of the mistake). If the mistake relates to the fundamental nature of the offer the contract can be voided.
Other possible remedies in equity include rectification (www.practicallaw.com/A36777), ordering specific performance (www.practicallaw.com/A36981) of the contract or rescission (www.practicallaw.com/A36811).