I liked Gregory Peck in "Boys from Brazil." I don't see that as a miscasting, whatever one thinks of the movie otherwise.
Middleaged Laurence Olivier and Greer Garson in Pride and Prejudice (1940), playing characters half their age. Every character in that film was also off, but that may have been writing/direction, not actual miscasting.
Pride and Prejudice (2005), also miscast in that the leads were too attractive. Lizzie is not supposed to be the most stunningly beautiful woman in the room and Darcy should appear to have an arrogance that requires £10,000 a year to compensate. That said, Knightly hit the role, MacFadyen not so much.
But the real miscast was Mr Bennett, who has taken to his library and sarcasm in refuge from reckless marriage to incompatible spouse. His one concern is to guard Lizzie from making the same mistake he did as a foolish youth, 20 odd years ago. And they cast 70 year old Donald Sutherland!
Speaking of Robert Mitchum being cast waY BELOW his age in the Winds of War film, my wife’s favorite miscasting was Frank Sinatra and Robert Mitchum in the 1955 film NOT AS A STRANGER, when they were 37 and 39 respectively, and played
MEDICAL STUDENTS.