California Doubling: The underwater scenes are supposed to be Gibraltar, but were shot in the Bahamas; also, since Japan didn't allow for Stuff Blowing Up in a national park, parts of the Little Nellie battle were shot in Spain.
The Other Darrin: Donald Pleasence takes over the role of Blofeld from the Anthony Dawson/Eric Pohlmann combo that had previously been playing the part...
The Other Marty: ...however, he wasn't the first person who attempted to do so. The original actor they cast was Jan Werich, who proved completely wrong for the role and so was replaced by Pleasence.
Serendipity Writes The Plot: Mie Hama was having trouble learning English, while Akiko Wakabayashi had much progress. Since the former took this so seriously that she threatened to kill herself if fired, the producers just traded the roles between actresses, as Kissy appears much later in the story and has less lines.
The Toyota 2000GT wasn't supposed to be a convertible, but it got converted into one due to Sean Connery's height.