I got a desperate call about a site being down, this is ordinary for me (advertisment: you can contact me if it happens to you). But the error I saw was new to me. This is surprising -- I have thought I have seen it all and then some. The modules/views/includes/plugins.inc was fataling about the function views_include not existing. At first I thought opcache went south cos how on earth could an include be loaded when the module isn't?? But it wasn't opcache.