Thousands of people were running - but the London Marathon's official fundraising page wasn't.
Virgin Money Giving crashed on the morning of the race - with last-minute donors who tried to log on greeted by a message saying the service is "currently unavailable".
The website also suffered a short outage on Saturday night because "lots of donations were being made" - but the organisation had insisted the technical issues were resolved.
Thousands of runners have relied on the Virgin Money Giving website to raise cash for their chosen charities, and many would have been hoping to benefit from a race-day surge in pledges.
"Virgin Money Giving are aware of the performance issue and the IT team are looking into it," a spokeswoman said.
The website was later updated to read: "Because so many amazing people are donating to their friends and family, the site is currently really busy.
"We're sorry for the inconvenience, please try again in a few moments. Thanks for your patience!"
Dozens of exasperated donors asked the website for updates on Twitter.
One runner wrote: "At prime time for donations, you have really let me down, I am sorry to say. Hope I haven't lost too many donations as a result."
A record number of competitors tackled this year's marathon, with the Duke and Duchess of Cambridge - as well as Prince Harry - cheering the 40,000 runners as they tackled the 26.2-mile course through the capital.
Hundreds of police were deployed to keep the runners safe, in addition to the estimated 760,000 spectators who were expected to line the streets.