Some 15,000 people who thought they had secured London 2012 Olympics tickets in the second round of sales have now been told they will not receive them.
The disappointed fans had been able to book seats for events which had already sold out because the booking website was not updated quickly enough.
Some 2.3m tickets went on sale online on Friday for 10 days on a first-come, first-served basis.
Locog said almost 90% of those who applied on Friday got some tickets.
Those who lost out fell foul of the way the Ticketmaster system was updated during the second round process, BBC sports news correspondent James Pearce says.
The original online ticket system was designed for the ballot arrangement used for the first round sale of tickets where live updating was not needed.
On a first-come, first-served basis, where tickets to events are withdrawn from sale as they sell out, the organisers had to rely on manual updating by a team of operatives.
These would appear not to have kept up with the speed of tickets sales, our correspondent says.
One applicant, Andy Pritchard, from Bangor, Gwynedd, contacted the BBC to say his application for second round tickets had been rejected.
"I was on the application website bang on 0600 on Friday to make my application and thought I had got tickets to three sessions," he said.
"As they were supposedly first-come, first-served, I had some confidence that I had been successful. But today I received an e-mail saying I had been unsuccessful and will not be going to the games, which I just found galling. Now I have no tickets for myself or my son."
A London 2012 spokeswoman said: "Over 150,000 applications have been processed since Friday for around 850,000 tickets. Just under 90% received tickets, subject to payment.
"Around 10% have not been successful due to the massive demand during the first two hours of sales where 10 sports sold out, some within 15 minutes.
"E-mails are being sent to applicants today, and whilst more applicants now have tickets to the Games, we know that there are still some disappointed customers and we will do everything we can to get them to the Games."
In all, 24 sports were in the second round, 18 of which sold out by Friday evening. Boxing and weight lifting joined the "sold out" list on Saturday.
Tickets for 310 sessions went on sale, 44 of which were medal events.
There were about 1.7 million tickets for football and 600,000 for other sports, including archery, hockey, football, judo, boxing and volleyball, among other sports.
Some half a million tickets were priced at £20 or less. A further one million tickets were priced between £20 and £50.
Tickets for football, volleyball, wrestling - freestyle and Greco-Roman - remain available.
Further ballot coming
Locog said there would be further opportunities to buy tickets for a wider range of events.
"Over a million new tickets will be offered to the British public next year from contingency seats, once venues are tested and licensed, and we aim to get as many of these tickets as possible into the hands of customers who have missed out to date."
Notification is being sent within 24 to 48 hours of applying. Payment will be taken once the sale closes at 1800 BST on 3 July.
The London Organising Committee of the Olympic Games (Locog) said: "Broadly speaking, those who applied first, on Friday morning, will be those who hear first."
Fewer than half of those who applied got tickets in the first round, as demand outstripped supply in many events. In the men's 100m athletics final more than one million tickets were requested.
Those who were successful in the first ballot, held from May to June, will get their own second chance to buy from 0600 BST on 8 July to 1800 BST on 17 July. However, a 2012 spokeswoman said there was no guarantee that tickets would still be available by then.