There are a number of journeys on the rail system in London where it is not possible to tell whether you’ve travelled via zone 1 or not. If the consensus is that you will have done, because the alternative is too convoluted, then the including zone 1 fare will be charged. But if avoiding zone 1 is deemed as likely as going through it then generally the avoiding zone 1 fare will be charged.
One example of this is when using the Overground Circle. Shoreditch High Street is in zone 1 with Hoxton on the zone 1/2 boundary. If your journey starts at Clapham Junction going clockwise to Canonbury it is charged zone 2 only even if you go via Surrey Quays; while anti-clockwise to Dalston Junction it’s zones 1-2. If you have to change trains at Clapham Junction (eg Denmark Hill to Gospel Oak) then you’ll need to touch the pink validator there, but that doesn’t work if the journey starts or finishes there.
A few years ago Geoff Marshall made a youtube video about another anomaly between Kew Gardens and Highbury & Islington. The assumption is that you use the Overground via Willesden Junction, but the system can’t tell if you use the District Line to Victoria and then change to the Victoria Line. It also works from Gunnersbury, and from Richmond (but not using South West Railway).
Last year I pointed out how to ride on the Battersea Power Station extension without paying for zone 1. This may get fixed next month, although the journey is so much longer they may just leave it there.
And so to the new one. I was alerted to the existance of this loophole by the TfL Journey Planner. Clapham Junction is again at one end of the journey while any Northern Line station north of Camden Town is the other. The avoiding zone 1 fare is defined by changing between the Overground and Underground at Camden Road/Town. However, due to limitations in the way interchanges are stored on the Oyster system it also works if you change at Clapham High Street/North. Note that when travelling towards Clapham Junction using Oyster (as opposed to contactless) you will be overcharged on exit at Clapham North. As long as you haven’t reached the daily cap for your travels your fare will be adjusted in the overnight back office process.