If you travel from Sutton (or further out) to Whitechapel or beyond using Southern to West Croydon and then London Overground you will probably be overcharged. This is because the oyster system decides that you are likely to have used First Capital Connect to Farringdon followed by Hammersmith and City to Whitechapel. This issue was first highlighted by Barry Doe in RAIL magazine earlier this year.
If you only go as far as Whitechapel then you should have avoided zone 1 altogether, but even if you travel to or through Shoreditch High Street you will find that the automatic fare has had the NR+Underground in zone 1 supplement of £1.10 added.
There is a useful workaround to this problem, however. If you hop off the Overground train at Canada Water, touch your oystercard on a pink route validator, then hop back on (or wait for the next train), the system knows you haven’t used Farringdon or the H&C and thus gets the fare right. Quite how the ordinary man in the street is supposed to work this out is anyones guess.
The long term solution would be to install pink validators at West Croydon, but the danger is that that will just confuse people further.