Because of differences in reporting periods between my employer and Centrelink, they decided I had a debt of $66.
This would have been cheaper and easier to just pay out, but I couldn't leave it be, so I fought their claims and was vindicated.
The stress of having to do this, especially where the level of potential debt wasn't stated initially, was significant.
The trivial amount that the system determine I *might* have owed should have been sufficient to decide that this case was not worth pursuing, but they still tried.
After significant difficulties with their self reporting systems and other requirements, I was able to get one of the Centrelink staffers to look at my case properly, and they determined that I didn't actually owe any money in the first place.