Later, the internet was filled with ‘Apple Fail’ posts. However, it is NOT true that the Face ID failed.
What really happened?
As you may already know, in iOS and many Android devices as well if your biometric security system fails too many times, you are required to enter your passcode. That is exactly what had happened during the event.
During the launch event, when Craig Federighi was giving the demo of Face ID, the phone was being handled by people backstage, and they did not realize that the iPhone X was trying to recognize their face. It could also be possible that the phone had been restarted before the demo. (Source:- Yahoo)
So, Face ID did not fail.