Negative inventory almost always comes from one habit: invoicing an item before its receipt is entered. The sale posts today, the bill for the same goods lands next week, and for the days in between the file shows you selling stock you never recorded receiving.
The usual sources:
- An invoice or sales receipt entered before the matching vendor bill or item receipt.
- A receipt dated after the sale it was meant to cover, so the sequence is wrong even though both exist.
- A purchase that was never entered at all — goods received against a verbal order or a missing bill.
- An item set up as inventory partway through the year, so early sales predate any starting quantity.
- Assemblies or builds sold before their components were received and built.
If any of these sound familiar, the item is likely already negative on your reports. A free bookkeeping health score flags below-zero quantities in a few minutes; the fix below is what closes them.