It turns out it's actually not all that hard to get a confession out of an innocent person. The same high-pressure psychological techniques meant to wear down a guilty suspect will make a lot of innocent people confess to something they didn't do. Since innocent people are more likely than criminals to waive their right to remain silent, they're put in a high-stress situation where they're not even clear about what they're being charged with. They also may feel guilt for some unrelated reason (they saw the crime and failed to report or stop it, for example). So, they say whatever they need to say to make the interrogation end.
For instance, one popular interrogation technique has the interrogator give a monologue claiming he already knows the subject is guilty, and then follows nine scripted steps to get a signed confession. It's incredibly effective -- it gets guilty suspects to confess nearly 84 percent of the time. Oh, and it gets innocent people to confess approximately 43 percent of the time. Add hinting at fake evidence, and you can up that false confession rate to about 94 percent.
"You ever wonder if maybe all these head games and lies get in the way of, y'know, justice?"
It's hard to argue with success! Especially if you're behind bars.