A bug in the latest version of FaceTime that lets someone listen in to your phone just by calling you - and it works if you don't pick up.
The bug was discovered Monday afternoon and lots of folks, including myself, tested it out - and it worked.
Basically, if you call someone on FaceTime, then add yourself to the call before they pick up, their phone is answered automatically, letting you listen in on what's happening around their device.
Apple has since disabled group FaceTime on their end, but until there is a permanent fix, I recommend turning off FaceTime completely. You can do this by going into settings, then FaceTime and toggling the switch so it's no longer green. Read my full post on the issue here.