Notably tapping in at another station refunds the charge and starts a new journey.
This appears correct at Underground station barriers, which have separate touch in and out units on either side of the barrier to distinguish, and were of course the original basis of the design. At an apparently late stage it moved on to the two-way units you find at unbarriered DLR stations and some other points, I understand purely as a cost saving move to avoid putting in two readers and the expense of laying in a power supply cable - this also accounts for some of the sub-optimal positioning of the readers, which are in different places at different DLR stations and not necessarily "in your face" (London City Airport station was a particularly bad example of this for years). These depend on logic for working out whether you are touching in or out. For those who only use mainstream Underground stations with a full gateline, possibly you have not experienced DLR stations with just one reader and the number of occasions when it is out of order - which is not advised to the DLR on-train ticket checking staff I discovered.
Ken Livingstone, who became Mayor during the development, stated in a radio interview I heard that his overall instruction to the development team was not to go for absolute perfection, which would take for ever, but to get the basics all up and working. While this is a very reasonable approach, and needs stating firmly on a number of rail and other projects, it doesn't quite gel with the progressive attitude, developed in fairness under subsequent Mayors, that anyone who appears not to have done everything perfectly is a criminal and gets fined the "Maximum Fare". Before anyone says it is not a fine, that is what all the general public passengers call it. It also doesn't gel with the sheer number of software logic updates that have had to be made to the system to deal with technical issues that were generally those not envisaged by the developers. The Out of Station Interchange logic took a particularly long time to get right apparently.
I was "maximum fared" I discovered on us all being ushered quickly out of Marble Arch station where the fire alarms had gone off and all the gates opened. I returned home from somewhere like Leicester Square, apparently the system could only auto-complete if you return later from the same station.
I suspect that Contactless is better at it than Oyster, depending on a big computer server to work out overnight what you may have done, rather than Oyster having to determine it as you go along at each unit.
tfl.gov.uk
