I don't know why I've been thinking about this lately, but I have.
Dios ex machina: According to wikipedia - "god out of the machine"; is a plot device whereby a seemingly inextricable problem is suddenly and abruptly solved with the contrived and unexpected intervention of some new event, character, ability, or object.
I guess it comes from those ancient Greek plays where a god is suddenly introduced to resolve the plot.
Some authors have used this and it has worked out okay for them. Examples: Lord of the Flies, when the wild kids are somewhat randomly rescued by the Naval Officer. Harry Potter and the Chamber of Secrets, when the Phoenix swoops in and saves them from the collapsing basilisk chamber. Or perhaps the most famous, The Return of the King, when the eagles fly in and rescue Sam and Frodo from the burning mountainside.
Sometimes it works, usually it doesn't. Who hasn't wondered why Frodo didn't simply fly on an eagle and drop the ring into Mt. Doom in the first place and cut out 9/10ths of the book(s)?
Most often it's called "Writer Cop Out."
Anyway, it gives us writers pause to make sure everything that happens in our story happens for a reason and doesn't feel contrived. All plot points must flow in a logical chain of events that makes sense based on our characters and their reasoning and choices.
The guy over at Moody Writing has written a few good posts on this lately: here and here.
Any other good examples of this?
Any good tips on avoiding it?