Trey Coolier shared a Google Webmaster Tools notification he has personally not seen before on Google+ for Google notifying him that there was a possible outage on his site.
This message is not new, we've mentioned it at least a couple times in the past. But what does it mean?
Well, it means when GoogleBot tried to access your site, it couldn't really access those pages. It may mean the pages are soft-404ing or it could mean your site was offline or more likely, there was a database issue with the site when GoogleBot tried to access a specific page.
We've probably all had this on our sites at some points, even I was able to look back and fine one:
If you see a ton of these, over and over again, don't ignore them and do something. Talk to your host, talk to your developers and make a change.
Google's John Mueller commented specifically about this case and he said:
Glad you found it useful! This particular one is a tricky case because based on the HTTP result code it wouldn't be seen as being down (so I imagine most website checkers wouldn't notice). A soft-404 could be just a bad setup in general, but it could also be that your database is returning error messages as content with a "200", which we might even index like that. If this happens sporadically, it's usually really hard to spot.
Forum discussion at Google+.