Citigroup's (NYSE:C) consumer banking business is getting it into trouble again — and this time the problem isn't mortgages.

The third-largest bank's messy and disappointing fourth-quarter report included an unexpected $500 million reserve for legal and regulatory issues "related to U.S. consumer businesses," in the words of Chief Financial Officer John Gerspach. It also appears to be related to areas scrutinized by the Consumer Financial Protection Bureau, which last year took enforcement actions against three big credit card companies over their marketing of payment protection add-on products and related practices.

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