Around 200 people who thought their only experience of the London 2012 Olympic Games would be minor heats of synchronised swimming have received an unexpected upgrade to the men's 100m final following an embarrassing ticketing mistake.

The London 2012 organising committee (Locog) confirmed on Wednesday that a decidedly unsynchronised error in its ticketing process had led to four synchronised swimming sessions being oversold by 10,000 tickets.

Locog and its ticket agent Ticketmaster spent the Christmas period contacting ticket holders and offering them alternatives, starting with tickets for sports they had applied for but been unsuccessful.

The upgrades have come at no additional cost, with Locog not charging for any discrepancy between the synchronised swimming tickets and the alternatives on offer. It has also offered to repay any discrepancy if the new tickets cost less than the originals.

Locog said the replacement tickets would come from a stock of unsold seats across the Olympic Games and that it was doing all it could to make good the error to fans who bought tickets in good faith.

Locog said the error occurred in the summer, between the first and second round of ticket sales, when a member of staff made a single keystroke mistake and entered "20,000" into a spreadsheet rather than the correct figure of 10,000 remaining tickets. The error was discovered when Locog reconciled the number of tickets sold against the final layouts and seating configurations for venues, and began contacting ticket holders before Christmas.

Thousands of ticketholders have already struck lucky, upgrading to tickets for major events including swimming and athletics finals, and in around 200 cases the hottest ticket of all, the men's 100m final.

Locog said the decision to award 100m final tickets was taken because one of the synchronised swimming sessions was on the same afternoon as the event, August 5.