McKellen confessed later on: “There was a question as to whether I might take over from Richard Harris but seeing as one of the last things he did publicly was say what a dreadful actor he thought I was, it would not have been appropriate for me to take over his part. It would have been unfair.”
Mel Gibson, Gladiator
Although Mel Gibson's best-known role currently is "the guy who revealed that he's a raging anti-semite and got laughed out of Hollywood," he definitely missed a big opportunity in the early stages of his career.