During the London 2012 Olympics an input error in a spreadsheet led to 10,000 oversold tickets for the Synchronised Swimming event. The person responsible for inputting the figures hit '2' instead of '1' indicating there were 20,000 tickets remaining, instead of the actual 10,000. Organisers had to contact ticket holders and provide tickets to alternative events.
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Estimated loss: £500,000
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https://www.telegraph.co.uk/sport/olympics/8992490/London-2012-Olympics-lucky-few-to-get-100m-final-tickets-after-synchronised-swimming-was-overbooked-by-10000.html

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