Interesting discussion!

Here's some more details of what happened, in answer to some of the questions above.

I think the crew had noticed the stray passenger in CE before the plane was moving, but the doors were locked and we started taxiing before the error was resolved. So I think this is where things went wrong. The plane then stopped before the runway while the error was sorted out - but this was after the safety briefing.

The extra passenger was sitting in the row behind me, which is how I know he'd checked in late and there had been what the purser called "a systems error".

Agree that all the information is on the video - but the purser walking around checking boarding passes while the video was playing meant people in CE weren't focussing on it.

I assure you this actually happened - I have better things to do with my time than make something like this up!