On 9 December I was travelling on a Network Railcard discounted paper day travelcard but absentmindedly tapped in with my credit card (which I normally use for commuting) at Canary Wharf Jubilee Line. I realised the error immediately and tapped straight back out again, then re-entered using my Travelcard.
It shows on my journey history, weirdly enough, as a pink reader tap at 12.30 at Canary Wharf (LU) followed by a yellow reader tap at 12.31 at Canary Wharf (LU), with a message underneath saying "Automatically completed journey. We have no record of where you touched in/out. The missing information has been completed based on a recent journey you made. This is to avoid charging you the maximum fare. Please remember to touch in and out to ensure you pay the advertised fare."
The charge is £0.00.