THE GUINNESS BOOK of World Records and the National Enquirer may find freaky lawsuits titillating, but to legal scholars, there are few things worse. They hate to see major legal principles originating in bizarre disputes that will never be repeated. Which is why the old legal saying persists that "Hard cases make bad law." And which is why your average lawyer persists in trying to "distinguish" unusual cases by harping on nitpicking details.

Marvin M. Mitchelson isn't your average lawyer. He's the fellow who coined the term "palimony" and then wrote it into the law books by winning big bucks for a woman claiming to be the ex-lover of wealthy actor Lee Marvin. Mitchelson's forte, in other words, is legal derring-do, and he recently sounded his call to arms again in an eyebrow-raising sex case involving a female Harvard professor. But this time, he probably won't win.

Mitchelson's client is Lee Perry '64, an assistant professor of Education who has been teaching psychology and counseling at the Graduate School of Education for five years. She's suing Richard C. Atkinson, chancellor of the University of California at San Diego and former head of the National Science Foundation, saying he was guilty of "fraud and deceit" and "intentional infliction of emotional distress" during an affair he and Perry allegedly had between about 1976 and 1978. Atkinson denies any liason.

Specifically, Perry says Atkinson impregnated her in mid-1977, talked her into having a wrenching abortion, and then broke his firm pledge to impregnate her again. What's more, he's predicting a "precedent-setting" verdict in Perry's favor that he says will encourage other betrayed lovers to undertake similar suits. A victory for Perry, Mitchelson says, would also deter men from making false promises to mistresses to escape embarassing predicaments.

If Perry is telling the truth--and reporters covering the story generally believe there was some affair between her and Atkinson--her case has strong gut appeal. In Mitchelson's words, her million-dollar suit says, in effect: "I lost a baby because you deceived me, you lied to me." At 38, Perry may well be too old to bear her first child, according to her attorney, who says having her own child has always been one of the psychologist's major desires. If Perry's facts are right, it's dead wrong that Atkinson--who is 53 and married for 30 years--could get away with duping Perry into aborting her fetus, all the while knowing he'd never keep his pledge to impregnate her again.

But Mitchelson has two major obstacles to success. The first is, as he acknowledges, that victory would require some court to create a new "right" for mistresses. Judges are almost always ginger about expanding legal rights--even when strong gut feelings are involved, especially when freaky facts are involved. The right would presumably be one allowing mistresses to be free from a lover's coercion in deciding whether to bear a child.

But Mitchelson himself allows that "No one else has ever sued for a right quite like this, and Harvard Law School expert Frank E. A. Sander calls Mitchelson's assertion "totally novel and rather questionable." Adds Sander: "It would be a novel--to put it mildly--notion to say that someone can claim damages by the right to be impregnated."

An uncharacteristic legal blunder by Mitchelson could also harm Perry's case. He disclosed to several reporters last week that his client would actually drop her case if Atkinson would again impregnate her, artificially or naturally.

Atkinson was flabbergasted by the request, and so, it seems, was Perry. This Monday she hastily convened a news conference to say that she no longer would settle for impregnation; Mitchelson had spoken wrong. The end result: Mitchelson looked like a headline-grabber, not an innovative attorney, and his client looked positively flaky. Wire service reporters have taken to calling her case the "no-deposit, no-return" lawsuit. Other monikers are less subtle.

Perry could still win her case, of course, for there's no substitute for having the facts and in-court legerdemain on one's side. But more likely, her case will prove once again that even an extraordinary lawyer needs a flawless game plan--and the wind at his back--to add new contours to the law.