I just got a penalty fare today. I had 80p on my oyster pay as you go, and the barriers in wimbledon let me in through. But when I was in zone 1, the ticket checker said I am not allowed to travel through zone 1, but instead use overground. But my destination being Stratford(zone 3) and the minimum fare being £1, I would have been in negative figures anyway. I paid £2(zone 3 to 3 via 1) when I eventually finished my journey.
I can't justify my fault for the penalty fare cause I would fill up my negative balance anyway when I top up next time, and there isn't any notice stating anywhere stat