Writing satisfying plot twists is also a challenge. They need to be the right kind of twist, deepening the story. Darth Vader's big reveal at the end of The Empire Strikes Back wouldn't have meant much if he had turned out to be Han Solo's father, since Luke and the audience had no emotional connection to Solo Senior. (Perhaps he'll show up in next month's movie.) Since Luke's goal was following in his father's footsteps, finding out his father was his nemesis was much more dramatic. Twists also have to arise organically from what previously happened in the story. It's not satisfying if they come out of nowhere, and that doesn't play fair with the reader either. There ought to be clues embedded in the story. Fortunately, if they're not in the first draft, they can be seeded in later.
One of my favorite plot twists that I wrote occurs in Twinned Universes (though I can't obviously reveal what it is). What's your favorite plot twist, either one you wrote or one you read? Feel free to share in the comments.