I think this is not uncommon to eInk screens. I've owned eInk readers from Sony and Amazon and they all occasionally had and/or have this happen. I wouldn't worry about it unless it becomes frequent. It is such a common thing that the Kindles have a screen refresh shortcut built in so you don't have to page forward the page backward. But with the newer readers like the 650 the speed of paging is fast and it is no longer an issue to do the page forward then page backward refresh.