Ten thousand synchronised swimming fans who thought they would be watching the event at the London 2012 Olympic Games will miss out, after an administrative bungle.
The London Olympics organising committee (Locog) sold 20,000 tickets, double the amount of seats available for the event.
Organisers put it down to "human error".
"As a result of finalising the seating configurations in our venues and reconciling the millions of Olympic and Paralympic ticket orders against the seating plans for around 1,000 sporting sessions, we have discovered an error in seats available in four synchronised swimming sessions," said a Locog spokesperson.
"In December we contacted around 3,000 customers who had applied for tickets in the four sessions during the second round sales process. We are exchanging their synchronised swimming tickets for tickets in other sports that they originally applied for."
This means some of those affected will get the chance to watch the athletics men's 100m final in the Olympic stadium, the most popular event of all.
Locog, which is headed by ex-Olympian Lord Sebastian Coe, will put a final 1.3million that are still left on sale in April, once the final seating details for each venue is known.
Critics have attacked Locog's ticketing system as unfair and favouring those with enough money to bid for lots of tickets.
Hundreds of thousands of people were left disappointed when the ticket lottery closed, while others were given tickets to multiple events.
April's release of tickets will be on a first-come-first-serve basis.