I set up Auto Top up and - when I returned later to Bond Street - I
checked at a machine that everything was in order. To my surprise, although Auto Top Up had
worked, I found that I had an unresolved journey. The Bond Street ticket office put that right
(and charged me £1.70 instead of what I think should have been £2.00). However, the ticket clerk
said that it had occurred because I hadn't swiped in at Acton. Now I *had* (the barriers were in
use and obviously opened), so I wonder what could have gone wrong? This is my first "hiccup"
with Oyster, though.

Sometimes if you miss touching in or out on one journey and make another
journey soon after (say within 2 hours) it gets confused because it can't tell
which entries belong together and that might give the result you experienced.