Betting exchange Betfair has voided millions of pounds worth of in-running bets traded on the Christmas Hurdle at Leopardstown because of a "technical error".
A single punter appeared to be offering to accept £21m worth of bets at in-running odds of 28-1 against Voler La Vedette, even as the 13-8 second favourite crossed the line in front.
In all, bets totalling £1.6m were staked, meaning a potential payout of tens of millions of pounds when the horse won.
In theory, the individual could potentially have lost £600m had his full limit of £21m been wagered.
Betfair suspended the market in order to investigate, and after lengthy discussions, it said the problem was due to a technical error.
A statement read: "In accordance with our terms and conditions, all in-running bets on this race, both win and place, will be made void.
"Customers betting in-play on this race will have seen that Voler La Vedette was available to back at 29 (28-1) when the in-running market was suspended, and that a considerable sum was matched on the clear winner at that price.
"An investigation has revealed that this was due to an obvious technical failure which allowed a customer to exceed their exposure limit.
"We fully appreciate the dissatisfaction this will cause many customers, and apologise for a very poor customer and betting experience."
Betting exchanges allow punters to bet at odds set and requested by other gamblers rather than by a bookmaker. Members can make both "back" bets, which are normal bets on a selection to win, and "lay" bets, where the punter is betting against the selection.