Today there has been a sudden flurry of '01' codes on ticket gates. It's the code for 'bad data'.
The reason, apparently, is that paper 16/17 photocards are no longer valid, so gates reject the associated Oysters.
It seems that the regulations of the Oyster system change on an almost daily basis - often with precious little information or warning to staff, let alone to passengers. It's hard enough for us to keep up with the flurry of amendments, yet LUL seems to expect teenagers to be fully up to speed with its ever-changing system.
It just adds to the chaos of a system which seemed like a bright idea but whose implementation has been a nightmare for staff.