I have a frequent issue regarding the Web Page display in Aperture, after upgrading to Mountain Lion. The web pages will occassionaly become blank when viewing the web page layout within Aperture (not uploaded web pages). Blank, as in a white page with no content. The photos are still visible in the browser, but not in the viewer. The layout usually returns after quitting and re-opening Aperture, but occasionally I have to restart the computer to fix the problem. This may happen several times an hour when working on Web Pages. I assume this is related to the Mountain Lion upgrade since I didn't ever experience this problem prior to the upgrade (but may be a coincidence).