Introduction -- Mental incapacity -- Non est factum in English law -- Common mistake in English law -- Mistake as to identity in English law -- Mistake as to terms in English law -- Mutual mistake in English law -- Mistake under the PICC, PECL, DCFR, and US Restatement (Second) of Contracts -- Misrepresentation -- Duress -- Undue influence -- Unconscionability -- Undue influence and unconscionability under the PICC, PECL and DCFR -- Conclusion.