Readers like to be surprised. A novel is a voyage of tension and intrigue: Will he find his way back to his true love or not? Will she recover from her broken relationship or sink deeper into her depression? A great writer knows how to finesse the action and the character's perspective so that the reader sees that there is more than one possibility for the character. Fiction without this possibility is predictable—and boring.
Still, an ending should be a natural outcropping of the action that has come before it. Sometimes, endings can be so shocking that the reader doesn’t recognize where it came from. If Julie broods over the loss of her teenage son for sixteen pages of a short story, you can't have her simply clean out his room, purging her life of his belongings on the last page. Julie needs to show some attempts to climb out of her grief before that moment, even if only in thought. Similarly, Daniel can't simply decide to quit drinking after a seven page long bender. He has to run up against the idea and struggle with it first.
Twist endings don't give the author an excuse to tack on an artificial or arbitrary ending. How frustrating to get caught up in the action of a story only to learn it was all a dream—or a drug induced hallucination, or in the imaginings of an insane character. Equally as upsetting are the endings where something or someone comes from out of the blue to rescue the main character from a seemingly impossible situation. Joe's about to be kicked out of school when he gets a letter saying his Great Aunt Lulu died, leaving him her fortune. Olivia is chased through the park by an attacker and just when he's within arms reach, her boyfriend and his football buddies round the corner. (In fiction, this is called "deus ex machina," a Latin term meaning "god from the machine.") And we can't forget the end-of-story death or suicide. It's certainly a shock to see a character put a gun to his head and pull the trigger in the last line, but it's not always right for the story.
So it comes down to this: twists and surprises won't work if they're trickery. If the reader's left out of the fun, the joke is on them. O. Henry, perhaps one of the best known writers of the twist ending, could offer up quite a surprise without making the reader feel duped. In "The Gift of the Magi," Della and Jim are short on money as Christmas approaches. Della sells her hair to buy a chain for Jim's watch. She soon learns Jim bought her the hair combs she'd been eyeing—and he had to sell his watch to do it. The story is told from Della's perspective, so the reader learns of Jim's choice when Della does. The information isn't hidden from the reader, so it doesn’t feel like slight of hand on the part of the author. Here's another example from O. Henry: In "The Ransom of Red Chief," two criminals hold a young boy for ransom. The boy turns out to be such a difficult prankster that the criminals end up paying his father to take him back. That the criminals' intention—to get money—is turned on its head is quite a twist. It grows out of the action of the story and the characterization of the young boy. Readers are surprised, but not tricked.
The success of the twist ending is all in the execution of it. Hopefully, this look at what makes such endings work will help you decide if your group is right on the mark in their suggestions or if it’s time for you to move on.