The below two images are quite interesting. The image on the left gives us the impression of having 5 holes and 1 bump. The image on the right looks like it has 5 bumps and 1 hole. What makes this a fascinating optical illusion is the fact that these are the exact same image.
That's right these two images are identical, one is turned upside down. The simple act of turning this image upside down completely changed our assessment of the image.
The reason for this is that the brain assumes that light comes from above. For centuries the best light source was the sun (came from above). Now we have various sources of light but one thing most of them have in common is that designers typically put all major light sources above us. This is a very strong predisposition on our part. Thus we view shadows and light with the assumption that the light is coming from above.
Another example can be seen below. The top image seems to be a large dome, could this be a lava dome preparing to erupt?
Credit: D. Roddy (LPI)
Here we think of this as an interesting effect and no more than that. But analysts viewing pictures from other planets often analyze the image from the wrong perspective. Many times these pictures end up in science magazines and journals (one example here) completely upside down and misrepresented. Go to this page to read more about the problems that some analysts have in getting past this optical illusion. Scan down the page and you'll see many examples of errors in astronomy reporting and analysis.