If you travel by bus and go onto any route and at the front of the bus you see the destination of the bus displayed.
Lets say take the 26, last stop would be Waterloo. If you go on and that destination is shown as Waterloo but the drivers asks you to go off and jump onto another bus anywhere between where your journey began and Waterloo, then you must ask the driver for a transfer ticket or you are going to be charged again for the journey when you jump onto another bus. unfortunately not all bus drivers give that information when they ask passengers to go off early but they should.
Once you obtained the transfer ticket, wait for the next bus, show the driver your transfer ticket, to continue the journey without paying twice for it.