I got caught fooling the barriers last year. Forgot to buy my travelcard, saw the huge queues at the ticket booth on the platforms and doubled up with my partner to get through the barriers. Unfortunately a revenue protection officer was stood on the other side disguised as a biker and caught me bang to rights. He chose to ignore the mitigating factors that I had to get a travelcard to continue my journey to Swiss Cottage anyway, and that I'm famously forgetful, which is fair enough I suppose as I could have been lying :) Anyway, they prosecute and take the case to court. I pleaded guilty in my absence and got a couple of hundred fine. Don't risk it!