Found an "interesting" problem with Oyster today. One of my collegues
has been over from the States and goes back today. He had a prepay
oyster which originally he bought with cash but he topped up with a
credit card.

Because he had used both cash and topped up with a credit card he
couldn't get a refund (well they would post a cheque to him).
Fortunately there was someone at work happy to buy it off him for
cash.

Tim.