A great poster is both eye-catching and informative, effectively conveying the feel and tone of the movie it's selling and persuading you to part with your hard-earned cash once said movie opens in cinemas.
They can be pieces of art in their own right, and we've seen some truly gorgeous ones over the years.
But, because we're so used to giving them quick glances while walking through a multiplex, or skimming over them while browsing an article, a lot of the hidden details and secrets found in certain movie posters can easily go overlooked.
Most of the time, we don't even expect posters to harbour any secrets at all. We know they're there to sell a movie - which is where the real surprises will lie - so posters are often given no more then a passing "Eh. Looks cool." and no further thought.
However, with today's zoom-and-enhance-obsessed internet culture, where streams of commenters pore over every single frame and image a movie has to offer, secrets were bound to be revealed at some point, right? Here are ten of the sneakiest that you probably missed...
10. Gremlins (1984)
Gremlins is a classic family movie that's stood the test of time and is revered to this day, and funnily enough, its movie poster hides a secret referencing another classic family movie that's stood the test of time and is revered to this day. Imagine that?!
Look closely at the above poster, paying attention to the button on Billy's jeans.
If you squint, you can make out the logo of Amblin Entertainment, the company that produced Gremlins and E.T.
The logo is a nod to this famous scene from the latter film, showing a bicycle flying through the sky.
This is very hard to notice by just looking at the poster normally, so it's not surprising it took years and years to unearth.