I sometimes have a page flashing once, and sometimes it looks like it flashes twice when I turn a page. Sometimes I have the same effects when panning around a zoomed page. But so far it didn't bother me that much. It would be interesting though to know why that is. I don't really think that it's a bug. Maybe it's a feature ?!?
(Ok, just joking) I think it has to do with the programming, but I don't know what triggers this second flashing. I'm not sure, but I think my 505 doesn't do this.