Got a call from one of my wifes consultants that her site was down. She was just the first - the phone went off the hook pretty quick. ALL of the subsites off of my multisite came up with blank white screens. Down, nothing, notta. A tad freaked I called WPEngine to help me out. Luckily these guys are pretty savvy. They came to me with a PHP fatal error
[Thu Mar 17 06:27:23.954985 2016] [:error] [pid 29404] [client 18.104.22.168:36332] PHP Fatal error: Call to undefined function is_user_logged_in() in /***/***/***/***/wp-content/plugins/pro-sites/pro-sites.php on line 1799
Took down the entire system.
Network Deactivated the plugin and our world came back up.