Some people claim to see a rat in this famous NASA picture (below). Some even claim that NASA brought the rat on Mars to see how long it would survive.
Interestingly, the thing that strikes me first is how these boulders are aligned. They don't seem to be spread randomly. I'm less impressed by the rat, though I also see it.
I believe both are illusions. The boulder pattern can easily be explained by the laws of randomness. What about the rat? Can probabilities show that such a feature is not really surprising. How would you go about building a probability model to explain the 'rat'? The first part to the answer is that any of a billion of possible patterns could look like an incredibly unlikely feature, the one-in-a-billion chance pattern: what if instead of a rat, it was shaped like a bottle? Or like another animal? Or a fruit? Or like a human tool? Or shiny like gold? There are so many possibilities (a billion) that it is likely that one of them - any of them, in this case the rat - will emerge.
Chances are that under deeper inspection, you'll find a few more weird creatures in this image. For instance, I see a broken bone just above the bluish rock at the bottom. And I'm curious to know why this rock is bluish, unlike other rocks in the neighborhood.
Or what if the image was fake and the rat added later in the picture? That's also a real possibility. Here again, image forensics, another data science technique, could help answer the question.