Two weekends ago, 40,000 to 60,000 Oyster card users had their cards corrupted when they tried to use them. The problem was corrected in about four hours and Transport for London had to reissue new cards the following Monday to affected users.

ZD Net UK web site reported 60,000 users had their cards disabled, while AccountancyAge said 40,000 cards stopped working. ZDNet’s reports that Transport for London has launched an investigation to try to find out what happened.

“We’re currently investigating the cause of the incident with Transys (Oyster’s supplier),” a TfL spokesperson told ZDNet.co.uk. “It’s an ongoing investigation.”