Bank of Scotland has been fined £4.2 million for failing to keep accurate records of the mortgage payments of 250,000 Halifax customers.
The Financial Services Authority said the problem began in 2004.
The BBC reports the problem was due to the bank keeping mortgage records on two separate computer systems, without always updating them. It was discovered last year.
Bank of Scotland was part of the HBOS group - along with Halifax - before HBOS was taken over by the Lloyds Banking Group in 2009.
FSA director Tracey McDermott said that in a complicated organisation where several legacy systems exist, firms have to make sure they are synchronised, otherwise their customers suffer.