Next is where "very carefully" comes in: You must maintain credibility and consistency.

The story should agree with what we know about our universe, what we think we know, or what we might someday know.  When it doesn't, the writer should be able to provide a plausible explanation for the discrepancy.

This is where my previous caution about playing fast and loose with science - or other facts - comes in. For example, what if you read in a story that a shuttle craft flies west past the Eiffel Tower, crosses the Seine, and lands on the Left Bank?  What would it do to your enjoyment of the story when you discovered that the Seine divides Paris into north and south areas, so that the "Left Bank" is the south bank? The author should have done a little research to avoid the error, because it will be pretty difficult to come up with a plausible explanation for this discrepancy.