Have you ever tried to watch a movie and just couldn't get into it because you found the casting so ridiculous? Pam Anderson playing the president of the U.S., for instance, or Denise Richards playing a nuclear physicist (the latter actually happened)? The Fug Girls have documented 16 of those examples - I think my favorite is Freddie Prinze, Jr., playing Fred in Scooby Doo. One that I found really distracting that wasn't on the list (probably because it wasn't a huge part) was Mike Myers playing a British general in Inglourious Basterds.
Link via Go Fug Yourself
And Mace Windu was one of the few GOOD things about the prequels. I adore him and his purple lightsabre, and that both his presence and the color were because he /damned well wanted them/.
Gwyneth Paltrow as Rosemary in Shallow Hal (and as Ex Mrs Iron Man Psycho Bitch in Iron Man 2).
Renee Zellweger as Bridget Jones (they really could not find a chubby blond chick in London, could they?)
Uma Thurman in Kill Bill trilogy (turn off the volume to torture your eyes. "Look at me, I'm acting!")
But Keanu Reeves? Why they felt the need to take a chain-smoking, BLOND, BRITISH mage and turn him into a monotone Los Angeleno, I'll never known.
And don't even get me started on Shia LaBoof as Chas Chandler.
And I had my reservations about Will Ferrell, but thought he did a good job.