http://news.bbc.co.uk/1/hi/england/london/7071356.stm

This (and perhaps other similar?) scams is presumably what lay behind
TfL's decision to change the design of the Bus Saver ticket earlier
this year.

Before Oyster PAYG, I was a big fan of the Bus Saver tickets, and they
can still come in useful when travelling on the bus with an Oyster-
less visitor from out of town. Nonetheless by their very nature
they're vulnerable to fraud.

I wonder how many Ticket Stops (i.e. newsagents and other shops
selling TfL tickets) bought these forged Saver tickets from these
crooks to sell on to punters - I'm minded to think that quite a few
did, given that it was an easy earner for them. I guess that one way
to counter this fraud would be for TfL to threaten to expel shops from
the Ticket Stop scheme should they be caught selling forged Saver
tickets, which is such an obvious suggestion I wouldn't be surprised
if they've already done just that.