Meltdown 2002 - David Bowie sound-checking with 'Ashes to Ashes' which he didn't play in the concert later on - it was quite a great moment to watch (hidden away in one of the boxes on level 5) him singing in an otherwise almost empty auditorium.
October 2003: watching Concorde's last flight from RFH level 5 balcony with lots of RFH staff, which was quite a historic moment on a beautiful & sunny afternoon! 
July 2004 - Bang on the Hall with Evelyn Glennie, part of Rhythm Sticks
Evelyn Glennie had been commissioned to devise a unique new work and this involved local community groups, using the RFH foyers and its content as a massive battery of percussion. This was quite an extraordinary and fun project already and during one of the rehearsals we'd arranged for press photographers to come along, but Evelyn Glennie didn't have any drum sticks to hand for this photo opportunity and I had just bought a pair for my brother (who's a drummer and a massive Evelyn Glennie fan!) which I could lend to her for the photo and tell my brother that his pair of drum sticks had actually been used by his 'hero' for the photo shoot and part of the rehearsal.
13 May 2005 - Theatre of the New Ear
I was using the personnel lift for artists and staff that evening in order to go up to level 5, squeezing into the lift just before the doors were closing, not noticing that I had positioned myself just in front of Paul McCartney, who was going backstage to meet the actors before the beginning of the performance. He had to get off at level 3 and only then did I notice him, as he asked me very politely if I needed to get off here, too and smiled at me.