Google also gets soft 404 if you have many pages with very thin but similar content. I have a case of image gallery being a popup in a smaller window, where the popup page html only contains 2 iffames. The first iframe shows the big photo and the second iframe contains thumbnails strip. The page itself is set to noindex (but allows follow) to allow Google to get to big image..
Google is, however, reporting soft 404 for every of pages that contain 2 iframes.