[gknoy]

I used to experience weird "I know there's something there but can't see it" (e.g., words, my hand) at particular angles from the center of what I was looking at. It turned out that my blood pressure was so low that I was very near to fainting. It was especially bad on days when I was dehydrated, which was the big clue. Once I started drinking water much more often, the experience hasn't happened again.

I realize there are people experiencing migraines and other painful experiences, but if you have occasional "I can't see right / my head hurts" experiences, consider seeing if drinking a lot more water helps. ;)