If you are paying with a contactless device, make sure you have enough battery to complete your journey. If you do not touch out because your device has run out of battery, this will show as an incomplete journey, and you will be charged the maximum fare.
If you do not have a valid ticket for your whole journey, you will be liable to pay a full fare or penalty fare.
The penalty fare is £100 plus the price of the full single fare applicable for your intended journey. If it is paid within 21 days, the Penalty Fare is reduced to £50 plus the price of the single fare applicable.