PicLens has had a Windows-only version for awhile and recently added a Mac version. The Windows file was approved and the Mac version was still pending review, yet on the public display page, the Mac version was showing up. Because the file hadn't been copied to the mirror, it was returning a download error. The Mac version shouldn't show up if it's not public.
I also have an extension where the linux version has been released but the windows version is still in the sandbox, however the green button shows as if it is available. https://addons.mozilla.org/en-US/firefox/addon/5764 When you click on the green install button from windows firefox produces a dialog: Firefox could not install the file at https://addons.mozilla.org/en-US/firefox/downloads/file/23623/zinc-0.6.3.0-fx- win.xpi because: Invalid file hash (possible download corruption) -261 This is because the supplied file is an html redirection page for accessing the sandbox rather than xpi itself, but the xpi installer doesn't handle this.
That'll be fixed in Zamboni: bug 558237.