There seems to be a bug in the reading menu navigate function, I can't remember if this was in 2.3.1 or not:
While reading an epub, I bring up the reading menu, select navigate (<-->), and use the slider or arrows to go to another part of the book. Then I press the back arrow to take be back to my starting position. But it doesn't take me back to the correct page, rather a few pages ahead of the correct page. I have tried it with zero margins with the same result. The bug doesn't seem to affect kepubs.