Last week we found over selves heading to London Euston on the London Overground network, with Oyster cards as our ticketing method. On arriving at Euston we found that our Midland train has docked onto one of those platforms with nary a Oyster card reader or even ticket gates. Annoyingly we were forced to find another platform with ticket gates in order to validate the journey, lest we get charged the maximum £5.50 per journey with no daily journey caps.
The staffers there were well aware of the issue but they just shrugged while helpfully touching-out our cards. I wonder how many people unintentionally paid more than they were supposed to due the lack of signage and campaign by TfL. You may be able to scam them tourists, but not us.