A housing society incorrectly reduced a resident's monthly maintenance bill due to a billing error. When the error was corrected, the society not only added the difference but also charged 21% interest on the wrongly credited amount. Advice suggests that if the society's bylaws don't explicitly permit charging interest for such errors, the resident is likely not liable to pay it, as the mistake was the society's. Options include discussing a waiver with the managing committee or lodging a complaint with the Deputy Registrar of Cooperative Housing Societies or RERA.