Strange Cargo:
It was a very strange movie, and there's a story connected to it:
As a reigning female star of many years' standing, Crawford had a "first billing" clause in her contract, meaning that no other actor's name could be displayed as prominently as hers in a movie's credits or advertising. Crawford and Gable had worked together in several movies when he was not (yet) as big a star as she, so her "first billing" clause always applied. But by the time Strange Cargo was made, Gable had become the biggest star in the world, and he had been given a first-billing clause in his contract too. Since Crawford had experienced some box-office slippage by '40 (her last really big box-office year was '37, though her movies still made very good money), she was expected to yield gracefully and allow Gable to be co-starred with her, his name appearing as prominently as hers, and first, because of his professional standing at the time. Joan did no such thing.
She held both M-G-M and Gable to her first-billing clause, which she was able to do because hers pre-dated Gable's. Gable's name and billing came second to hers on the movie and its ads. Whatever satisfaction Joan got out of the situation, she did not benefit in the long run; Gable never made another movie with her, and he also stopped seeing her - they'd had a long-standing affair.
Joan would have done well to take a hint from Norma Shearer, who had yielded her own first-billing clause on The Women so that Crawford could be co-starred; Norma was known to be a tough cookie about stuff like that (and was no friend of Crawford's besides), but she was also a team player, and she understood how important it was to The Women's box-office success to be presented as an "all-star" picture. Norma even yielded a second time on The Women; in the middle of shooting, Rosalind Russell faked illness until her contract was adjusted to give her full star rights, which meant Norma was asked to sign a contract waiver for Russell as well as the one she'd signed for Crawford. Norma's professionalism on The Women was what made Russell a full and unquestioned star after years of supporting-player status.
Not coincidentally, M-G-M stopped putting its full weight behind Crawford about this time; her movies got less budget and promotion than they had previously. After a few years of this, Joan realised that M-G-M was basically through with her, and asked for a release from her contract. It was granted, and that's when she signed with Warner Bros.