What Makes a Good Plot Twist?

Sometimes a plot twist is the ingenious solution to an insolvable problem. Many times your readers aren't as stupid as your characters are, and they're going to guess it a mile away. In John Cleaver 5 that I just turned in, the first draft of it, I sent it out to my readers and one of them wrote back saying he figured it out like three chapters earlier. So I had to do that exact thing and come up with enough feasible alternatives instead of just the character being an idiot.