Got an overpayment notice, what now?

After 9 weeks of benefits I got a notice saying I must repay some of it. The letter mentions an appeal window and a repayment plan. Has anyone disputed one of these?

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

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.