Overpayment letter from the unemployment office - first time posting

The agency says I reported earnings incorrectly, but I think I followed the instructions. 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?

Thanks for writing this out. I went through something close to it 8 months ago, and it settled down once I kept a simple log of every call and form.