There's no way a Betfair client would have £600m+ in their account. Maybe £20 or even £50m from the massive syndicates who regard(ed) Betfair as safer than any bank, but not £600m. So the error has to be something technical. However, rumour has it, a helpdesk reply (not gospel, naturally) has implied it wasn't a Betfair issue.
I would be extremely surprised if this race was paid out in full - if it is a massive customer Betfair have offered credit to (has been offered in very selective cases), then the customer is quite likely to argue technical error that Betfair should have blocked and refuse to pay up. As mentioned above, I severely doubt it's a customer/syndicate with that much in cash reserves, and the other point to remember is that Betfair TSE Ltd are now under the regulatory control of Gibraltar authorities, who don't have a history of putting the punter first. Betfair aren't the kind of company who will take the whack on the chin if it was an internal balls-up, and £23m will make a serious dent into their all-important share price.