The gaffe means that the London Organising Committee of the Olympic and Paralympic Games must now ask thousands of people to return their tickets.
Ticket buyers will be offered spaces at their second or third choice of event instead.
Because many of the events that will be offered in exchange cost more than tickets to watch synchronised swimming – the cheapest tickets to the Aquatics Centre event are just £25, whereas the cheapest tickets to the main stadium cost £75 – many buyers will find themselves upgraded to more sough-after events, and at a discount at that.
A London 2012 spokesperson said: “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 1000 sporting sessions, we have discovered an error in seats available in four synchronised swimming sessions.
“In December we contacted around 3000 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.”