Avoiding Deus ex machina endings should go without saying. If you’re unfamiliar with that term, literally it means in so many words that God intervenes. If you have built a faith-based story in which this is a credible outcome, then that’s okay. However, it’s not acceptable for other types of fiction.
Other than the literal translation, it refers to an ending where something or someone comes along out of nowhere, like some sort of literary superhero, and takes care of wrapping up the story. These are annoying to say the least. As an author, if you get your characters in such an incredible mess you can’t fix it more appropriately, then you need to go back to plotting with your thinking cap securely in place.
If you write sci-fi or fantasy, you have a bit more freedom provided you set up the possibility for a dramatic ending beforehand so you don’t spring it on the reader out of the blue. Miracles are fine, as long as you develop the possibility beforehand.
Marcha's Two-Cents Worth 