Take particular note if you use machines at Victoria, Liverpool Street, London Bridge, and King Cross London Underground stations as these have had the highest number of complaints.
BBC London discovered the amount of refunds from a Freedom of Information request. So far 2,421 complaints have been made on over charging. Don't expect any errors on the machines to be magically refunded to your account as TfL explained that it will not be aware of any over-charging until the issue is reported by a passenger.
However they said "But we constantly check the screens on ticket machines and how the ticket machines are set up to minimise these problems."
Let us know if you've been over-charged recently and how you went about getting your money back.