Bill Cosby made a movie so bad in 1987 that he publicly asked that nobody go to see it!

Leonard Part 6 was a parody spy movie in a similar vein to The Naked Gun series. Cosby starred in the film as well as wrote the story and produced it. In the weeks leading up to its release, Cosby denounced and disowned the film. 

The film was so bad; it received several Golden Raspberry Awards, which are the antithesis of the Academy Awards. The movie was called Part 6, because the idea was that parts 1-5 were being kept under lock by the CIA. In the movie, Bill Cosby is brought back by the CIA to save the world from an evil vegetarian. He floods the villain’s base with Alka-Seltzer while riding an ostrich.

Cosby blamed most of the problems of the film on the director. He said that it was the director's first time and was too young and inexperienced to take on the film. The movie was a box office flop grossing on $4.6 million. Production cost of the film was $24 million! 

(Source)