Ann: FY19 Guidance and Outlook, page-4

Its just an accounting thing, not necessarily errors. When testing for impairment, the total profit, cash flow, or other benefit that's expected to be generated by a specific asset is periodically compared with that same asset's book value. If it's found that the book value of the asset exceeds the cash flow or benefit of the asset, the difference between the two is written off and the value of the asset declines on the company's balance sheet.

5his could just be old assets not being used anymore to drive profit.