A list of the London stations where passengers were "overcharged" the most when using Oyster cards has been revealed.
Waterloo saw the most "overcharge" payments, with a total of £2.5m last year.
In total, £61.8m of these payments were taken from passengers in 2010.
Transport for London (TfL) said any passengers who felt they were wrongly charged could apply for refunds.
The problems arose when barriers were not working or left open, or if cards were not read when swiped at the start or end of their journey.
The maximum £7.40 single fare is deducted for all "incomplete" Oyster journeys and it is up to the passenger to ensure his or her card is swiped.
But when there are large crowds at stations, TfL will open the barriers and implement a system called called "autocomplete".
Users will have to touch in at the same station within three days or they receive the maximum fare.
The list of the London stations where passengers had to pay the most incomplete journey charges was obtained through a Freedom of Information request by the Lib Dems.
A total of 14 million journeys in 2010 involved possible "overcharging".
Since Oyster cards became accepted on national rail stations at the beginning of 2010, rail passengers have paid up to £25.8m in incomplete journey charges.
Passengers using the London Underground paid even more of the maximum fare deductions - up to £30.1m.
In addition, there was up to £3.2m on the Docklands Light Railway and up to £2.5m on the London Overground.
"This level of overcharging is totally unacceptable," said Lib Dems London Assembly Group leader Caroline Pidgeon.
"It is clear that not enough is being done to ensure the machines and the technology are working properly."
A TfL spokesperson said: "Maximum fares are not overcharges.
"We have maximum fares in place to ensure customers validate their journeys or we would open to widespread fare evasion across our network."