If you tuned into the Apple keynote event on Tuesday night, you may have noticed a little glitch in Apple’s grand event. When Craig Federighi, Apple’s Senior Vice President of Software Engineering was demoing the new iPhone X’s FaceID feature, it failed at the first attempt. Federighi held the phone up against his face and the iPhone X promptly failed and asked for the pass code. Everybody noticed, but Federighi very smoothly stepped past the glitch. But as was inevitable, next morning people were asking questions, and Apple was of course forced to answer.
According to what Apple told Yahoo Finance, the feature didn’t actually fail after all. An Apple representative told the publication that the iPhone X locked down because the phone was handled by a lot of people before the event and each time, the iPhone X looked for the face of Federighi and failed. So it did what it always does in this situation. It asked for a pass code after failing to authenticate too many times.
The publication wrote:
People were handling the device for stage demo ahead of time,” says a rep, “and didn’t realize Face ID was trying to authenticate their face. After failing a number of times, because they weren’t Craig, the iPhone did what it was designed to do, which was to require his passcode.” In other words, “Face ID worked as it was designed to.

If you actually think about it, the same happens with the TouchID. If an unregistered finger taps on the home button too many times, the iPhone would go into lockdown mode, which could only be unlocked using the passcode. 
Having said that, it comes across as a shoddy job when Apple didn’t pay too much attention to this aspect of the feature, right ahead of the demonstration.