Revealing a hidden backstory can be very surprising. Say a character has been acting strange throughout the story, and then you suddenly reveal a traumatic event from their past that explains their behavior. This gives the reader new insights and surprises them.
Surprising the reader with the introduction of a new and powerful element late in the story can be effective. For instance, in a science - fiction story, a new alien species with unique abilities could be introduced near the end, completely changing the direction of the story and the reader's understanding of what's going on.
A great technique is misdirection. You can lead the reader to believe one thing is going to happen, like a character is about to get a big promotion at work, but then it turns out they are actually fired. It keeps the reader guessing.