Oh, twist endings? They're all about setting up one expectation and then smashing it. The classics usually play with the identity of the villain—the detective did it, the narrator is the killer, the victim faked their death. But what really gives a twist its bite is how fair it feels in retrospect. The best ideas plant tiny, almost invisible clues that seem insignificant until the reveal. I'd say an idea works if you can read it twice and have the entire emotional experience shift.
For a short story, you need a tight, closed circle. Maybe a support group where everyone is confessing to the same crime, but for different reasons, and the twist is that the facilitator is the only real culprit, orchestrating the whole thing. The limited scope lets you control the information perfectly.