[technomage]

Yes, this is unreliable in my hands, also. In the site preferences inspector, the problem images show as a text mime type if opened into their own tab.

I also notice that the 5.9.1+ sneaky peeks no longer display embedded video objects (e.g. .flv files) in the site preferences inspector so they can be saved/displayed. I would also call this a bug, as this feature is something I use quite a lot.