An event in a story that's exactly the reverse of what was expected is an example of?

Situational Irony.

Explanation:

Situational irony often appears when there is an incongruity between of expectations that something is about to happen, and what actually happens. In other words, the final outcome is opposite to what the audience is expecting. So there needs to be sharp contracts and contradictions in order to spot a situational irony. In that way, the viewer can distinct what is true and what is false, and imagionary.