Sometimes it is necessary to touch out at a station without actually making a journey.  The Oyster system reacts in one of three different ways depending on how much time has elapsed between touch in and touch out.

Between 0 and 2 minutes

This could happen if you immedately discover that there is disruption, or if the display on the gate says that you do not have enough credit to complete your intended journey and you decide to exit again to top-up.  The maximum fare will be left on the card and this will not count towards the daily cap.  However, if you re-enter (after topping up for instance) within 45 minutes the maximum fare is refunded and a new journey is started.  The touch out must be at a ticket gate (ie you had to touch out to open the gate).  The new journey can be at the same station or at another one, but you cannot use a bus or tram to get between the stations.  This is probably only useful in Central London.

Between 2 and 30 minutes

In this instance the system will charge the cheapest single fare from that station at the time of touch in.  Usually that will be a single zone fare, but it can be two zones if the station is the only one on it’s line in that zone.  This charge will count towards the daily cap.

More than 30 minutes

Now the system assumes that you have made two incomplete journeys and charges you accordingly.  Neither journey counts towards the daily cap.

These charges are levied to discourage fare evasion.  If there is a genuine reason for making a same station exit then you will probably be eligible for a refund of the extra charges.  If the station has full Oyster facilities (i.e. most Underground stations when the ticket office is open) then you may be able to get the credit immediately, otherwise call the helpdesk for assistance.