I don't normally need oyster, as the vast majority of the time I use a travelcard. However, on Saturday I found myself at Watford station after a walk, and saw that trains were heading to Amersham! Not wanting to miss the opportunity to do the north curve, I vended an Oyster and TfL took £10 of my money.
Once I was back in Watford, I couldn't refund my money as, at 1600 on a Saturday, the ticket office is closed. I then tried Watford High Street and found that Overground ticket offices can't refund Oyster.
So I am now £7 lighter than I wanted to be until I return to London on the 28th March.
Now I read that I will need to take my passport with me in order to get my money back. (I am assuming this is still the case)
Funny how they are much more willing to take money than they are to give it back.