An Oyster card glitch meant that Transport for London (TfL) lost approximately £250,000 in just one morning.
The problem was linked to software problems relating to a fare hike of 1.1% being applied to Oyster cards.
The BBC reported that around 100,000 free journeys are believed to have been made between 04:00 and 10:00, which cost TfL a quarter of a million pounds.
Barriers at many underground stations were left open, as commuters enjoyed a rare free journey on the network.
The fault was met with joy from commuters, who expressed their happiness on social media.
“Thanks @TfL,” wrote one passenger on Twitter. “I was expecting to pay the new 2nd of Jan fares in the tube but the whole oyster ticketing system is down! Free journey for me!”
Buses were also affected, with some drivers waving through passengers showing Oyster cards.
However, contactless debit cards were still working properly.
A spokesperson for TfL said: "This morning's technical issue with our Oyster Card readers has been resolved. Customers should now touch in and out as normal. Anyone charged a maximum fare due to this issue will get an automatic refund credited to their Oyster card early next week. We apologise for this disruption."