Writer's Fallout Shelter

Oh, I don't know about never. I get very suspicious when people talk about never. It would probably be better to say that one should use it very sparingly, and mainly by really top-tier high caliber writers can use it at all. It is still a bad idea more than ninety nine percent of the time.

(Are you asking for information or sparking conversation? Ah well, I'll just keep going.)

The term deus ex machina translates literally as "god from a machine," and it comes from the old Greek plays where this plot device would play out literally. A "god" actor would be brought onstage via machine, and would effectively put everything straight. Colloquially this is used to mean a plot device in which some higher power comes from left field, resolves the story, ends all the troubles, etc. It is inadvisable (at best) as a plot device because it is basically a cop out. The characters don't need to change/develop/solve their problems.