Masello's twentieth rule is a corollary to the previous one, and in fact I discussed much of this rule in my last post. But I do have a few additional thoughts.
With this rule, Masello advises writers to know the reality behind the place and customs they write about. He gives a few examples of stories that would ring untrue to people in the know, including a student story where the main character, a New York City secretary, spoke three languages, drove her Porsche into Manhattan every day, parked in the company provided lot, and then drove out and parked on the street to get lunch. I can certainly understand his disbelief; after all, it's almost impossible to get a parking space in Manhattan at lunchtime.
Now, this doesn't mean you can't justify something that would normally not be acceptable. To riff on the example above, perhaps the secretary is the rich boss's daughter, who has spent every summer in Europe and so knows other languages. The boss is trying to instill a better work ethic in her, so he's forcing her to work as a secretary at his company. She's rebellious, though, and she continues with her profligate habits. She parks in the executive parking lot (perhaps they have one) and she takes long lunches, not caring if it takes her hours to find a space, all in the name of her rebellion against her father.
I think most readers would accept that scenario a little better, but in the end, it's very much based on the unbelievable one presented before. And, of course, science fiction and fantasy readers exercise a frequent willing suspension of disbelief -- but the writer still needs to meet them halfway. Even if the territory is completely made up from whole cloth, because you're writing about the world of Faerie or the far future, you still need to know it well.
And the rewards for knowing the territory are obvious. Back in May, I had a post on Reader Identification, in which I quoted Lawrence Block on why readers strongly identify with real places in fiction:
"...the presence of real parts of our own real world helps convince us that the writer knows what he's talking about... The more I can accept the idea that the author knows whereof he writes, the easier it is for me to believe further that the fictional story he's relating is true -- and it is upon this voluntary suspension of disbelief that fiction depends for much of its power to move us."
A reader wants to believe in a made-up world as well. And the best way to help your readers do that is to believe in it yourself as you're writing it.