Got an overpayment notice, what now? (update)

The agency says I reported earnings incorrectly, but I think I followed the instructions. This is my first time dealing with any of this. I did not notice any errors at the time.

If you are unsure, free legal aid in your area may help for this kind of notice.

Thanks for writing this out. I went through something close to it 12 months ago, and it got easier once I kept a simple log of every call and form.