Got an overpayment notice, what now? (update)

I received a letter saying I was overpaid about $950 on my claim. 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 4 months ago, and it settled down once I kept a simple log of every call and form.