Tooth Fairy Rule

Among science fiction writers, there is a concept called the Tooth Fairy Rule, which states that you can only invoke a mysterious outside force (the tooth fairy, for example) to explain the unexplainable once in a story. Otherwise, you can't really expect to maintain suspension of disbelief.

Other writers are more strict, for example Isaac Asimov in the introduction to Asimov's Mysteries.

The above may be viewed a kind of rule of thumb.