Today I needed to travel from Euston Square (NOT Euston) to Leicester
Square.
I got on a circle line service to Kings Cross St Pancras to change for the
Piccadilly Line.
When I exited the circle line platforms, I passed through the barriers and
then again through the barriers for the Piccadilly line.
Because I passed through two sets of barriers, I have been charged £3
(Oyster Card Pre-pay) for a £1.50 journey.
Changing between sub-surface and deep level tube lines at Kings Cross isn't
a change I make regularly so can anybody explain to me if I have done
something wrong here (I just simply followed the signs marked Piccadilly
Line that took me through the barriers) or is this a problem that needs
resolving?

Simon