Reread by accident, repeatedly... (and you?)

I have a question for you: What book (or books) have you rereadby accident? That is, what have you reread only because you did not remember reading it in the first place? On a related note, how many times have you done such a thing?

I do this a fair bit, as I read *very* quickly and will often am about half-way through before it dawns on me that I've read it before...by which point I figure I should just go ahead and finish it. My record is Robert Heinlein's, Friday. I have reread it 3 times by accident [and once on purpose].

I read it as a teenager when it was released in 1982. I was in a airport several years later and saw a copy as a massmarket paperback. I read the back, it sounded good (and range no bells), so I picked it up. Several hours into the trip, I realized I had read it but zipped through to the end. About a decade later, I saw a "new" trade paperback edition (again in an airport) of a book called "Friday". It range a vague bell, but when I read the description, it didn't resonate. Again, somewhere over the midwest, I realized I had read it...and realized I had done this before . 

Finally, I was picking up some trip reading (there is, undoubtedly, a theme here with my lowered focus at/around flying) at a used bookshop and saw a 20th anniv. edition (or some such thing) for a book called "Friday" that range a vague bell...but not enough that I did not stand there and think, "how could I have missed this, it is right up  my alley and I've read so much bloody Heinlein". Again, well into the flight before I realized that this was the same tale that had hooked me repeatedly. 

I reread it in the last year or so...this time on purpose. That one doesn't count. Can you beat that number? I've read Friday 5 times....only twice on purpose. Sad. Very sad.