(CNN/Money) Some pension plans have overpaid retirees for years — now they’re demanding their money back.

For retirees, it can mean owing tens of thousands of dollars. And with little warning, their pension checks are being slashed to cover their debt.

In April 2011, New Jersey resident Carol Montague received a letter from American Water Works Co.’s pension plan saying it had overpaid her for more than five years and wanted its money back — plus interest. Montague, now 67, was told she owed roughly $45,000.

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