I had my second occurance of this in the past year. Noticed while reading that some letters were getting blocked out by a blurry spot that moved with the eyes. No other noticable symptoms. It was very disconcerting so I closed each eye separately, and noticed that the spot did not move, and was there for both eyes. The spot was small at first and seemed to have a prismatic effect, although clear it blocked vision in the area. Eventually, the spot started expanding, and disappeared in about 20-30 minutes.
After two days of thorough testing by the opthomologist, he found nothing. His only guess was a possible optical migrain, with no pain, or possible TIA. I'd be interested in what others might think.