Some optical illusions get more confusing the longer you look at them. Take the triply ambiguous object illusion as an example: At first glance, it looks like a fairly straightforward 3D object, but as the video below shows, what you see changes entirely depending on which angle you view it from.
The secret behind the "3D" object in this illusion is that it isn't really three-dimensional at all. It's a flat piece of paper with shaded, geometric shapes sketched onto it, giving it the appearance of depth. This means that shapes that seem to be at the "top" of the object appear at the bottom if you turn the paper 180 degrees. The flag pin, which stays in the same spot no matter how you view the illusion, is the only element that isn't two-dimensional.
The triply ambiguous object illusion from Kokichi Sugihara received first place in the 2018 Best Illusion of the Year contest, an annual competition held by the Neural Correlate Society. You can check out more award-winning optical illusions here.