Juli Clover at MacRumours:

There's a major bug in FaceTime right now that lets you connect to someone and hear their audio without the person even accepting the call. You can force a FaceTime call with someone and hear what they're saying, perhaps even without their knowledge.

We tested the bug at MacRumors and were able to initiate a FaceTime call with each other where we could hear the person on the other end without ever having pressed the button to accept the call. To exploit the bug, all you have to do is add your own phone number to a FaceTime call you've already initiated, which apparently creates a major FaceTime issue.

Adding your own phone number to Group FaceTime a second time causes the call between both parties to connect. You can hear the person on the other end and they can hear you, even though the call wasn't accepted.

What's weird about this is that I had an unknown FaceTime caller a few days ago. I've disabled FaceTime until Apple issue an update.