Most psychics who make a prediction will usually make the VAGUEST predictions they can that will subsequently be used to describe a broad range of events. Once something happens that fits that vague criteria, they then start going about as marking it as some sort of "achievement" that we should take notice of, so that we'll more likely heed their words in the future. The ones who make the most SPECIFIC predictions are the ones who essentially stick their necks out, waiting for the blade to drop, because if people actually begin to believe your words to the dot, and you're wrong, expect some serious repercussions. Here's an example: people give up their lives because of this so called "Doomsday" or "Rapture." They give up their possessions, their families, their jobs… and guess what? Nothing happens. Well, shit guys, we threw away our lives for some prediction that wasn't true! In Harold Camping's case, most of those people followed him for the second prediction, and still, nothing happened.
tl;dr --> Herp Derp, Harold Camping's predictions sums this up nicely.