Every eye has a hole in its vision, and your brain paints over it

Every one of your eyes has a hole in its vision, about ten Moons wide, and you have never seen it. It is where the optic nerve leaves the retina, a patch with no light-sensing cells at all.

Close one eye, stare at the cross, and slide the shape until it vanishes. Then watch your brain paint over the gap with whatever surrounds it, so convincingly that the world still looks complete.