So say that a customer was overcharged $100 in interest in Jan 2017 and they carried a balance all year. By Dec 2017 they carried an extra $100 on their credit

So say that a customer was overcharged $100 in interest in Jan 2017 and they carried a balance all year. By Dec 2017 they carried an extra $100 on their credit card balance needlessly so they paid >$20 in additional interest (APR 20%) due to this overcharge. The article only mentions returning the original amount AFAIK so it looks like Citi might still be profiting from this error.