(See: 'Girls Gone Wild' shirtlifter lawsuits)
Under contract law, to be legally bound to a contract you sign, you must have the mental capacity to enter into that agreement. Any level of intoxication obviates the signor's clarity of thought and judgment and invalidates the contract.-Legal.zoom story above.
Certainly there can be no rational ground for asserting that a man can have a moral obligation to obey a legal rule that does not exist, or is kept secret from him, or that came into existence only after he had acted, or was unintelligible, or was contradicted by another rule of the same system, or commanded the impossible, or changed every minute-- Legal Realism and the Social Contract
Fuller's Public Jurisprudence of Form, Private Jurisprudence of Substance