Negative inventory happens when QuickBooks records the sale of more units than it shows on hand — most often because a sales invoice is dated before the purchase that actually supplied the goods. It quietly distorts item cost and cost of goods sold.
The mechanics are the problem. When you sell into a negative balance, QuickBooks has to relieve a cost it doesn't yet have, so it estimates one — and when the real bill arrives later at a different price, it goes back and adjusts, scattering corrections across dates you have already reported. Under average cost this can throw the running average off entirely; you see COGS numbers that don't reconcile to purchases and an Inventory Asset balance that drifts from the count. It is common in businesses that invoice fast and enter vendor bills slowly. The fix is almost never deleting transactions — it's getting the dates and the order right so purchases land before the sales that draw on them, which is exactly what a negative inventory fix works through.