My dad sent me a link to the video below several months ago.
Since then, Illusion Chasers Susana Martinez-Conde and Stephen L. Macknik have singled it out for attention on their Scientific American blog.
As they notice, it is amusingly difficult to see what's going on here, even though it doesn't take us very long to understand what is happening. All the understanding in the world, it seems, doesn't work its way down to our eyes, or brains, to let us simply see what happening.
What makes this video particularly striking is that, unlike most of the famous optical illusions beloved of perceptual psychologists — the Müller-Lyer illusion, the Penrose Triangle, the Kanizsa Triangle — this is no 2D line drawing. Moreover, it is a display of real people and their movements. If there's one thing we are very good at parsing and making sense of it is human movement.
But here we are baffled.
Psychologists like optical illusions because, by showing how we mis-perceive, they bring out the principles that govern normal perception. It's the closest we can get to tinkering with the mechanism which is our visual system.
But it is helpful, too, to think of perceptual illusions, of this sort, not as break-downs, but as outright successes. By carefully camouflaging themselves, and organizing their movements, these dancers create new visual patterns. It is striking how very good we are at noticing and paying attention to these patterns.