This is the primary downside of reading without a bookmark. Any book I've read on public transportation, I would guess on average each page gets read about twice (with some chapter intros reaching double-digits). It's hard to figure out if it's really something you've read before, or if it's just a very logical continuation of what you read yesterday.

I also feel obligated to one-up you. One time in high school English class, the teacher put a sample student essay up on the projector and picked it apart in front of the class. It took me half an hour to realize that I had written the essay in question, and by that point I had concluded that I did not completely agree with it. I learned something that day about writing.