Race comes into both examples, but I don't think the disjunction is primarily a racial one. Jackson isn't just an urban African-American male: He's, like, one bottom-line, hard, intense dude. He doesn't really fit among Lucas' puppet creatures the way Ian McDiarmid's fruity Palpatine does. Similarly, Grace Jones was unsmiling angular ebony androgyny personified. She would have laughed a thirtyish Moore out of bed, never mind a sixtyish one.
There are deliberate disconnects, too: Charlie Kaufman pulled off a comic coup in Being John Malkovich when he had Malkovich (as himself) hanging out with Charlie Sheen (as himself). It was the ultimate "huh?"—especially since Malkovich had publicly lambasted Sheen as a sexual predator.
Can anyone top these? Give me two actors (or an actor and an object) who/that cannot be reconciled and threw you out of a movie. And if you want to make things interesting, try to think of two people who shouldn't have fit together on screen—i.e., Robert Redford and Barbra Streisand in The Way We Were—but had some strange alchemical connection anyway. ... 5:10 P.T.