My son stumbled across a little known oddity of making a same station exit last month. He touched in at Crayford and after a few minutes was informed that the line was closed because of a fault. He touched out and I drove him to nearby Slade Green where he touched in on the validators. Further along the line his card was inspected by a Southeastern RPI and he was told it wasn’t validated. Thankfully the RPI responded to my son’s protestations by scrolling back through the journey history and he noticed the Crayford to Slade Green journey. He decided to let my son carry on, and on touch out at London Bridge an incomplete journey was recorded. I called TfL the next day and they refunded the overcharge.
What actually happened was a mystery to the customer service staff at TfL, but it was escalated to Cubic who came back with the explanation. If a here-to-here journey is recorded (touch in and out within 2-30 minutes) a minimum fare journey is charged. If you then touch in at a validator at a different station within the maximum journey time from the first station this journey is changed to be between the two stations. It’s a revenue protection measure to combat fare evasion using a travelcard. The problem is that it leaves you out of the system when you should be in.
The problem only exists if you use a validator to touch in at the second station. If it’s a gate then it knows to start a new journey. If you use contactless the back office will correctly interpret your touches and charge you properly. There also won’t be a problem if you use a bus between the two stations.