Dartington Threesome
Something that promises to be good, but is in fact so disappointing it is positively injurious.
This stems from the case of a certain Mr X, a plumber from Hitchin, Herts, UK, whose former girlfriend offered him a threesome as a birthday treat - except that on answering the door in expectation of his partner's friend, the extra person - who was male (sub-optimal, but not in itself fatal to the project), turned out instead to be expecting a homosexual encounter as Mr X's girlfriend was proposing merely to watch whilst Mr X had sex with the stranger - ie "A Dartington Threesome" is an M-M but no F threesome...".