If I'm reading a book on my iPhone, switch to another app, then come back to iBooks after a while, the book will be on a page much further along than where I left it. It's incredibly weird and frustrating.
I expect that if I am on say, page 200, when I return to iBooks I will still be on page 200. But often I'll be on page 258 or something. I can get around this by manually bookmarking the page when I leave, then remembering to go to the Bookmarks section and resume that way, but this seems absurd. An e-book reader ought to be able to handle remembering where you left off!