Rectify positive account into a negative...

On the 24th of October I received an sms from African Bank indicating that a technical error relating to my credit card interest has been rectified. This transaction made that I went from a positive balance to an owing negative. I called yesterday and the agent imp**** (or this is what I understand from the reason given) that I unduly benefitted from interest that was not charged correctly and it was then rectified. What the hell is going on, are they charging clients to make up for other acquisitions...