Overpayment letter from the unemployment office - 12 weeks in

After 12 weeks of benefits I got a notice saying I must repay some of it. I would rather hear the honest version than the polite one. I did not notice any errors at the time. Who should I talk to first?

Keep everything in writing and save the confirmation numbers. It made the follow-up calls so much easier.

Collect your pay stubs and certification records for the weeks they name. Mine showed the error was a date mix-up.