Another kind of bad ending is the one where none of the characters have a role in the ending. The gods step in and the ending is a big bang up one that leaves the reader shaking their heads. This is generally not a problem for me since I usually know the ending before I begin the story and I also know the beginning.
When a writer comes up with this kind of an ending, and it doesn't necessarily mean the gods really step in. What it means is the writer can't find a way to end the story. They could go on and on past the resolution without having a resolution so they bring in something not planned for and there is an ending. The flood waters come and wash the characters away. A bomb drops and all the characters are gone. The writer dusts off his hands and says that's a wrap.
A wrap or a cheat. An ending doesn't have to be happy or sad. It must be satisfactory. For the ending the be satisfactory clues to the end of the story must be planted throughout the story. When the end happens the reader can sigh and say I suspected that's what would happen.
Leave the gods, the machines or the disasters our of the ending unless you've planted the seeds throughout the story. This will leave the reader feeling satisfied even if the ending is not what they imagined.