I have had four issues on Oyster.
1. I was unable to collect a topup due to illness. The help desk advised me, correctly, that it would be refunded to my card.
2 & 3 Delays took the journey time over the max. On both occasions I went straight to the ticket office (this was a long while ago) and it was sorted before I left the station
4. Unresolved journey. Changing from rail to tube at Tottenham Hale, in the crowd I didn't see any readers and assumed that entry through the tube gateline would be like boarding the Jubilee at Stratford. (That dates it). Again it was sorted as soon as I spotted it.