It's possible to travel between Clapham Junction and Highbury & Islington in two different directions: go clockwise through Willesden Junction, a route entirely in zone 2; or travel anti-clockwise up through Canada Water, a journey which is mostly in zone 2, but briefly dips into zone 1 when passing through Shoreditch High Street.
So... hang on. When you exit at Highbury, how does the Oyster system know which way round you've gone? Which journey fare does it deduct: all zone 2, or zones 1+2?
A friendly email (or three, it took them a while to understand our question properly) to Transport for London prompted a reply that might surprise you.
TfL is kind enough to assume that you have been the 'zone 2 only' way round. So whichever way you go, you'll be charged just £1.50 (off-peak) instead of the £2.10 you'd expect to pay for travelling through zone 1. Bizarrely, this means that it's cheaper to travel through the 17 stations from Clapham Junction to Highbury, than it is to make the five-stop journey from Shoreditch to Highbury along the same tracks.
This got us wondering... are there any other anomalies like this, with the Oyster and zonal fair system?
It also made us consider whether this is the furthest you can travel in London, on the lowest fare. Any off-peak journey that doesn't enter zone 1 stays at just £1.50 — if you travel from a zone 2 station to zone 6, it's that same 'flat fare' — it goes up to £1.60 if you venture into any zone beyond 6 from zone 3 (a zone 2 to 7, 8 or 9 becomes £2.70 for some reason).
We've worked out the best (and worst) value tube and bus fares on the network here. Let us know if you've found any other Oyster oddities in the comments.