http://news.bbc.co.uk/1/hi/england/london/7524754.stm

The Oyster system used on London's transport network has broken down two
weeks after another fault left 40,000 customers with corrupted cards.

The latest problem has affected pay as you go Oystercards on the Tube
network.

Barriers are being kept open at all stations until the problem has been
rectified, Transport for London (TfL) has confirmed.