Did you watch a whodunit movie with someone who later proclaimed that he knew the killer all along? Don't be annoyed with such people for they probably couldn't help it. They may be suffering from "hindsight bias":

Researchers found that they are suffering from “hindsight bias”, when a person genuinely believes that they know something when in fact they are hearing or seeing it for the first time.

Although the effects might seem relatively harmless, researchers claimed it could prevent people learning why something has happened or from taking advice.

Prof Neal Roese, of Northwestern University in Chicago, said: “If you feel like you knew it all along, it means you won’t stop to examine why something really happened.

“It’s often hard to convince seasoned decision-makers that they might fall prey to hindsight bias.”

I, of course, have known about it all along: Link