Another non-Trek example is that I've often wondered what comicbooks exist in the various universes of the comicbook companies. We know from several issues that characters like Superman and Captain America (Steve Rogers even was the artist on his own comic for a time) have their own comics licensed in-universe, but we're pretty certain that its not openly known that Superman is secretly this one mild-mannered reporter at a great metropolitan newspaper. So, how much is known about the characters personal lives and secret identities?...
There was a cute issue of CAPTAIN AMERICA many years ago that addressed the existence of the old b/w movie serial. According to the story, the real Cap agreed to the serial on the grounds that it would be good for wartime morale, but (as in real life) the serial changed his origin and secret identity because, of course, they couldn't give away Cap's real
I also remember a bit, back when the Incredible Hulk
TV show was on the air, where the real Bruce Banner was so upset to find out that his tragic story had been turned into a TV show that he Hulked out and attacked the set!