As part of David Bowie's Meltdown in June 2002, Coldplay performed live in the Royal Festival Hall.  Part way through the evening the group started to perform 'Yellow'. Just a few bars in, out of the darkness leapt a man on to the stage who decided to 'singalong' with Chris Martin.  There was a slight scuffle of Security on the stage, to which Chris Martin seemed to indicate to them to just let him be.  Chris Martin stood back and let the man sing the song (with full backing from the band).  At the end, the guy took a bow, waved to 'his fans' and left the stage.
I wonder if he's out there to submit his memory of this evening...?
The Royal Festival Hall always has an 'electric' feeling when the Hall has capacity audience.  Such a night was in May 2003 when Jessye Norman performed as part of Song on the South Bank.  She had the audience in the palm of her hand throughout the entire solo song recital.  After a standing ovation she performed a piece by de Falla.  With the piano introduction, she lowered her head, stamped her foot and then looked up at the audience as if a bull entering the ring - nostrils flared and widened eyes.  It was amazing!
In the Autumn of 2003 the South Bank Centre classical music team programmed a project entitled 'WorldVoice'.  Spanning three weekends from September to October , choirs from all over the world were invited to take part, and each one of them chosen because it represented the very best in its tradition.  Visiting choirs included those from Finland, Georgia, Japan, Russia, South Africa and Turkey.  For a number of the choirs it was their first visit to the UK.  As part of the project, the Education Team organised an event I attended called the big sing thing 2 .  It took place on the Ballroom floor of the RFH and was a global celebration of the voice, bringing together members of the public, some of London's community choirs (Georgian, gospel and Turkish) and some of the WorldVoice visiting choirs. These choirs included the Real Happy Singers from South Africa who taught us how to sing 'Isicathamiya' .  It was a day full of fun and a wonderful opportunity to learn a totally different way of singing.  An experience shared with many.