When editing an image in gthumb, I've many times lost time because of accidentally hitting the scroll wheel. In virtuall any other application, the scroll wheel is non-destructive: If you scroll up, you can revert to the previous state by scrolling down.

This doesn't work right in gthumb. When you've scrolled up, you get a dialog asking you to either save your changes, or discard them. "Undo"ing the scroll operation is then much more complicated than what it took to cause it. I have to save a temporary file, (selecting format information) and then re-open that  seperately, which in the process also discards all the Edit:undo history.

It seems that this feature was introduced in http://bugzilla.gnome.org/show_bug.cgi?id=131416 , despite some significant objections. With this one as well, I'd suggest it's important to reconsider what the mouse-wheel should do here.