A great movie mystery can turn a pretty good film into a cultural touchstone, inspiring debate across countless message boards, forums, and real-life water-cooler discussions.
Whether or not they're wrapped up in a movie or franchise eventually or left to hang for years before viewers finally figure them out, there are plenty of directors both in and out of Hollywood who thrive on including plots in their projects that pretty much require three viewings to understand.
However, it's often the little details that go into each and every production that make the eventual resolution of these mysteries so engaging, inspiring everyone to get back in order to spot all of the clues packed into the production. And the best of these are always the ones that make you feel a little dumb after the fact.
They're big enough that they give the game away as soon as you notice them, yet obscured enough so that you don't even question what they mean until the movie has already told you. As a result, whether you spotted them retroactively your first time through or took years to notice them, they all had viewers kicking themselves when they realised.