Peter Lawford was actually the first choice to play Bond. He turned it down. No doubt the biggest mistake in his career.
Apparently Richard Todd, who died recently, was also offered Bond in DR. NO but had a scheduling conflict. He would have made an interesting choice, Lawford not so much. But probably every Brit actor under 60 was considered for the role at one time or another.
Ian Fleming wanted David Niven to play 007.
The biggest mistake of his career was being the clean-up guy for the Kennedys.