I don't think that's the cause at all. I switch often between Book and Code view, where it would pop up the message if there were any errors at all when I switched to Book view. It's a pretty simple epub, only 4 pages so far (but it did it from page 1), and using only h2, h3 and p class=calibre1 and that's it. Not really too much that can get screwed up and nothing complicated like tables.
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