If you forget to tap out using contactless payment on public transportation, you will be charged the maximum fare for that service.
The system cannot determine your actual destination, because the system needs to know where you started and where you ended your journey, to calculate the correct fare amount.
When you don't tap out, the system just assumes that you have traveled to the furthest possible point that is on the route and charges you accordingly.
The system requires both a tap on and a tap out to register that you have completed your journey and to calculate the correct fare.
You can reach out to and contact the transit agency and explain the situation to them and they might be able to adjust the fare if you provide them with the needed details about your trip.
If you tap in on a bus in London and you forget to tap out, you will often be charged the maximum bus fare for the entire bus route.
And in some cases if you can demonstrate an incomplete journey or if the system has made an error, might be eligible for a refund.
For a train fare you will be charged the fare from the station you tapped in to the next station you tap in at.