When the harpist of the Royal Opera House failed to turn up to a performance of Swan Lake, the perfect stand-in appeared to be waiting in the wings.As panic reigned backstage, Koen Kessels, the musical director of the Royal Ballet, bravely stepped forward and decided to accompany the orchestra on the piano himself.However his impromptu performance did not go entirely to plan.Some audience members walked out, while others complained about wrong notes played on an out of tune piano.Mr Kessels grew up playing the piano, but by his own admission lacked the “genius” required to be a world-leading concert pianist. “I can be disciplined when I’m obsessive about something, but I wasn’t truly obsessive about the piano,” he has said. Conductor and harp stand-in KesselsCredit:JACQUES DEMARTHON/AFP “Koen Kessels is a conductor, he’s not a pianist, he’s not a concert pianist. He’s an orchestra conductor.“Getting a harpist in time wasn’t feasible. A harpist did come in but didn’t make it in time for the start.“The harp is a very distinctive instrument and there was just one in the orchestra. It’s not like a violin where having one missing we could recover.“In Swan Lake the harp has a moment in Act II.“Things like this happen rarely and normally you can get somebody in to cover, there are systems in place to get people in.“But in this instance there was difficulty getting someone in at the right moment.“It’s difficult getting someone across London in time. If somebody is unwell we usually have more notice.“It was a rare and extraordinary circumstance.” The piano he played is usually reserved for warming up performers rather than a concert grand.Some audience members reportedly walked out on hearing that there was no harp available and the part would be played on the piano.One member of the audience said that “those of us who stayed witnessed piano playing with lots of wrong notes, on an instrument in desperate need of tuning.” However, others praised Mr Kessels’s last-minute rendition. “That was the performance I loved best out of four Swan Lakes I saw during this run”, said another audience member.“Obviously I noticed, because the famous harp bit was played on the piano, but I still enjoyed the performance”, they said.A harpist was eventually found and arrived in time to play Acts III and IV. The confusion led to show finishing around 25 minutes later than planned.Mr Kessels studied at the Royal Conservatoire in Antwerp and made his conducting debut at the Royal Ballet with a 2008 production of The Nutcracker.He is the music director of the Birmingham Royal Ballet, and was appointed to the same post at the Royal Ballet in the 2015/16 season. Dancers on stage in the Royal Ballet production of Swan LakeCredit:Alastair Muir/amx A spokesman for the Royal Opera House said: “It was a particularly unusual circumstance caused by a scheduling mix up”. He instead chose to be a conductor, after a friend told him that conductors are “the only one who doesn’t play an instrument”.This was not the case in this performance on June 8, as Mr Kessels doggedly got through the celebrated harp cadenza and the violin solo with harp accompaniment.After Act I went ahead without a harp, the Belgian conductor was literally waiting in the wings to step in for Act II and played a piano on the side of the stage, as there was not time to get one into the pit. Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.