About 100,000 Enbridge customers are learning they owe hundreds of dollars to the gas distribution company.
Customers in Enbridge's budget billing plan pay installments based on 2009's gas consumption, but the gas company admitted it made a mistake and set some payments too low.
Enbridge said the miscalculation means its customers will be receiving notices this month telling them they need to pay anywhere from $400 to $1,300, or more.
Affected customers are being told they can spread this re-adjustment payment out over the next year if they need to with no penalty.
Enbridge gas distribution is part of the larger Enbridge company but is not directly associated with the current oil pipeline spill in Michigan.