Transport for London has wrongly charged bus passengers several thousand pounds because its Oyster readers have taken money from the wrong cards.
In late 2012 TfL began letting commuters pay for their bus journeys with a tap of their contactless bank cards.
If a traveller has an Oyster card and their contactless card in the same wallet, either one could be charged. If a passenger has paid for a season ticket on their Oyster and the bus reader detects their bank card instead, the passenger is effectively charged twice.
So far nearly 1800 travellers have been double charged, forcing TfL to hand over around £11,000 in refunds, according to figures obtained by the Metro newspaper through a Freedom of Information request.
The errors represent a tiny proportion of the six million journeys made on London buses every day but the figure is likely to rise significantly when contactless bank cards start being accepted on the underground and overground systems later this year.
TfL is already using posters, e-mails and public-address announcements to warn commuters about 'card clash' - where readers pick up a bank and Oyster card and rejects both.