The classic "Contracts" one is of course, MYLEGS for Statute of Frauds:
Marriage, contemplation of,
Year, scope of, can't be performed in,
Land, interest in
Estate
Goods, $500 or more
Surety--guaranty of the debt of another.
Another is SUM FIG, attacking enforceability of K:
Statute of Frauds
Unconscionability
Mistake, Mutual (Raffles v. Wichelhaus)
Formation--Offer, acceptance, consideration
Incapacity--minor, drunk, insane
Gangster--Fraud, duress, illegality