Got an overpayment notice, what now? - what worked for you?

I received a letter saying I was overpaid about $400 on my claim. I have read a lot of threads here and could not find my exact situation. I did not notice any errors at the time. Are payment plans possible?

Thanks for writing this out. I went through something close to it 3 months ago, and it settled down once I kept a simple log of every call and form.