Mainstream credits: A Clockwork Orange (McDowell, 1971), Lawrence of Arabia (O'Toole, 1962), Julius Caesar (Gielgud, 1970), The Queen (Mirren, 2006)
Adult credits: Caligula (1979)
Circumstance: Mainstream film turned adult film, unbeknownst to its famous cast

The story of how these four acclaimed actors ended up in what many consider to be an adult film is really the story of a film production that ran off the rails, with Bob Guccione, founder of Penthouse, at the controls. The four actors claim they did not sign up to do a sex flick—nor did any of them have actual sex in the movie—but there's no arguing with the fact that a Guccione-funded film directed by Tinto Brass was going to be, at least, extremely sexy. Brass was fired, original screenwriter Gore Vidal quit, and ultimately the film (four years in the making) was stitched together by Guccione himself with added footage of hardcore lesbian scenes. 

The actors claimed to have been shocked at the final product, although you have to think that Malcolm McDowell kinda dug it. To this day, he says that despite its notoriety the film does have its moments.