Dang, they just casually leave a phone call going (through their demo telephony system), then do some apparently unrelated demo where they fix a bug that is causing another part of the system to crash, and then as a big reveal, the phone call they left up is still going untouched.
The best thing that we weren't faking this. We did actually run though the whole sequence as was shown in the movie with having a call going while another part of the crashed and was fixed.
Also significant enough to be mentioned by Joe Armstrong himself: https://joearms.github.io/