I can relate to way too many of these anecdotes. I think I need more Si.
Two Tuesdays ago, I was running laps around the block and tripped over an unlevel surface in the sidewalk. Never mind the fact that I didn't notice it when I was running, I went about an hour before discovering I was bleeding and needed stitches.
Many times if I am reading something and it either bores me or I'm just too busy thinking of something else I can be "reading" the book until I stop and notice that I don't remember anything I had been reading for the past half hour. I'm actually reading the book and saying the words in my head, but my thoughts are focused entirely somewhere different while I'm doing it. Has this ever happened to anyone else?