Among science fiction writers, the Tooth Fairy Rule states that a mysterious outside force (the tooth fairy, for example) may be invoked to explain the unexplainable only once in a story. One cannot expect to maintain the all-important suspension of disbelief otherwise.
Some writers are more strict, for example Isaac Asimov in the introduction to Asimov's Mysteries.
The above may be viewed as a kind of rule of thumb.
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