There does seem to be a fault with the oyster system.

Today a customer with a Zones23 travelcard made a journey from Camden Town
to Finchley Central and was charged £1.70 for the trip. The price should
have been £1.00.
I asked the Prestige helpdesk and was informed that there is a problem with
routeing. I am unsure exactly what that means.

Those of you who use a combination of travelcard and pre-pay should keep a
close eye on how much you are being charged. Though I am sure you already
do.