There are a lot of interesting (and weird) cognitive "hallucinations" or illusions. Psychologists are interested in these illusions because they can use them as a way to investigate and understand the nature of our brain cognition and perception. There are a bunch of wild ones. One of my favourite one is the blind spot. We basically have a bundle of nerves going from our eyes to the brain. The area where these nerves exit is called the blind spot because we cannot see in that region. However, our brains trick us and hides the blind spots from us. Tricky brain! Check this video out to find your blind spot