IPhoto Forgets Recent Edits

Question

I am pretty experienced in iPhoto editing, but I have a very annoying problem that seems to come and go; I cannot pin down the exact circumstances that cause it. I have just done quite a number of edits to photos in a certain Event when, suddenly, the slider settings have gone back to their standard positions. The edits are still there, but the particular positions of the sliders have been forgotten. I was about to go back and tweak my edits a bit more, but now I cannot, at least not from the positions that I last set the sliders. For other edits that I've done on more recent photos, the positions of the sliders are remembered, even after quitting and relaunching iPhoto.
Has anyone else had this experience? In particular, in moving from one photo to the next when editing, I find it is very important to use the left-right arrows at the bottom of the screen. If I navigate by clicking on the thumbnail ribbon at the top of the screen, this is an almost certain way to lose the slider settings, but not absolutely always. This unpredictablity of behaviour is very, very frustrating.
Any ideas would be highly welcome.

Answer 1

Terence,
I have a science/engineering background, so I understand the process of step-by-step elimination, but I don't see how creating a new library is going to prove anything in the short term, because today I did some editing on shots I took in 2007 and everything is working fine. I thought that cropping photos before/after editing might be the source of the problem, but currently it works fine both ways.
I therefore went over my previous events to see which ones still had their edit history still intact, i.e. the sliders set away from the starting position. Broadly speaking, any edits I did in 2006 and 2007 have had their sliders reset. The photos still retain their edited state, of course. I found today that the movement of sliders survives closing an event after editing, closing and relaunching iPhoto and a computer restart. I assume therefore whether the fact that my older photos have had their sliders reset means I must have edited them in an earlier version of iPhoto (not v8 or v9).
Photos that I took and edited in 2008 and 2009 mostly have their sliders displaced, thus retaining their edit history. However a few have had them reset, including a few that I worked on in the last day or two. Of the photos of any age that I worked on recently, some have retained their slider history, some have not, so something nasty happened recently, but heaven knows what. My computer has been working fine and I have nothing but highly reputable programs running on it; besides, I thought all OS X programs were sand-boxed and so immune from one another. As I said, all is well today, so there's no way of finding what went wrong. Given that no-one else has contributed to this post suggests that my problem was possibly a one-off, although the evidence suggests that it has happened occasionally before.