Been getting messages in gWMT regarding "soft" 404s on a couple of sites lately.
Looked into them and was fairly impressed. These sites (2 of them) are completely custom, no off the shelf code, built from scratch. NO structured data, NO google Highlighter.
Google is identifying pages on the site that are of little use to the visitor, for example, maybe a product out of stock, or an option selected for a product that does not exist.
To me this would not be as impressive if ours was an off the shelf CMS or ECom software, something opensource that google could train the bot on. This is completely unique, custom to us stuff, and it changes a lot.