Wells Fargo acknowledged Thursday it improperly collected fee rebates that should have been given to a public pension fund while acting as its trustee. The bank said it made a mistake when setting up the rebates and said the $15,000 in fees have been returned to the Chattanooga, Tennessee, fire and police department pension fund. The bank made its statement to CNBC after The Wall Street Journal reported that the fund's board had grilled the bank last month over what it considered to be suspicious practices in its institutional retirement and trust unit. The board fired Wells Fargo as the trustee, the Journal said. It's the latest in a string of recent setbacks for the bank.