As a point of interest I signed up for the Gold card a while back, and used it only for the balance transfer rate, and have never used it for a purcahse.
I noticed I had a Retail Interest rate being charged over the last few months, and could not figure out why. (Normally charged a Promo rate for the balance transfer for life promo rate)
I have sent them an email to credit me the tiny amount (and it is about $2 per month).
I have my account paid by direct debit, just for the minimum amount. What I have found though is that even though the payment amount is credited to my account, it is often followed with a Late Payment penalty of $40. The $40 is credited back a day or two later, however I think this is where they have slugged me at the retail rate.
So if anyone has been charged a retail rate and hasn't used the card for normal purchases, it is easy to see. However, if you have a balance transfer, and use your card for purchases, it may not be as easy to see that you maybe charged a little extra interest on any reversed out late payment fee.