I was very worried when I first got the letter as I was not expecting this. The letter says I was overpaid and at the time I didn't even think to contest it, I just assumed it was correct and paid the amount. This happened just before Christmas, so not a great time to be hit with a bill. ~$500 may not seem like a lot, but for me it is.
The fact that they would trust a single algorithm and have practically no human oversight of sending out these debt notices is mind-boggling.