Too much pressure.
Too much influence.
Too much exploitation.
Notice of an induced mistake (misrepresentation).
Relationship of trust and confidence and a transaction calling for an explanation.
Proof of one party exercising too much influence over another.
Threat to breach a contract, which causes (on a 'but for' test) a party to enter into a contract when he has no other reasonable alternative.
An unfair bargain, a disadvantaged party and unconscionable conduct.