Having worked at GLA as a ground staff last year, it's surprisingly easy to miss the 'change the screens' step out, especially as a new start. There are many separate actions required in the boarding process!

You know you've done it when there are far too few bodies at the gate! Hugely embarrassing, but I did do this once myself. Rectified by lots of tannoy calls, and deep apologies offered to my panicked passengers running for the gate...