Got an overpayment notice, what now? - any advice?

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. Has anyone disputed one of these?

Keep everything in writing and save the confirmation numbers. It saved me later.

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