I had a strange experience with my sense of sight a couple of years back, and I've repeated the event occasionally since then with similar results.
I was walking on a sidewalk, I was quite tired and had my eyes fixed on the horizon and somewhat unfocussed. I thought I saw something wiggle on the sidewalk, so I looked down. There was nothing there. I continued, looking back at the horizon, and I saw the same thing again. The sidewalk was one of those that was made of large square slabs, concrete I guess. The seam between the slab in front of me and the one in front of that seemed to shimmer. I kept my eyes pointed horizontally, and the effect got more noticable the closer I walked to it. After a few steps, the seam left my field of vision, but I saw the same thing happening to the seam in front of it. If I looked down, the illusion disappeared, but if I stared at the horizon all the time I could see it. I kept my eyes up, and stopped. The seams stopped wriggling too. I rocked back and forth on my feet, the ripples moved when I moved.
I always find it an interesting sensation when I can directly detect the limitations of my senses - like finding my blind spot, or putting one hand in hot water and the other in cold water for a while then putting both in warm water. I think in this situation, either I was seeing irregularities in the shape of my retina, or I was seeing aliasing as a result of the limited resolution of the eye.