Talk about being poor predictors! It turns out that 54.8% of people who tell posters that they don't think they'll vote, end up voting. And on the other side of the coin, 13.3% of those who say they are "almost certain" to vote, don't show up at the polls. This matters a lot when it comes to predicting turnout of elections and it also reveals a type of prediction people often make about themselves and get wrong.
Tip of the hat to Politico for uncovering this gem. Here's the Hardvard study they source, "Why Bother Asking? The Limited Value of Self-Reported Vote Intention."