Citigroup for the first time has publicly detailed one way it dressed up its balance sheet and incorrectly hid risk from the public.

In a filing disclosed on Thursday, Citigroup explained how it made an accounting mistake that hid billions of dollars in debt from investors. Citigroup said it misclassified certain short-term trades as "sales," when they should have been classified as borrowings because the bank had demanded additional collateral from trading partners when it wasn't supposed to.