This can happen in very rare, coincidental cases within a very specific, narrow window of time; unfortunately, the only solution is worse than the problem itself.
Our system adjusts your inventory stock quantity whenever we process a sale for a completed payment after checkout. However, it may be possible for two buyers to add a quantity to their respective cart orders at the same time which can each be met with your available inventory separately, but not both together. Thus, if the second buyer takes their order to checkout before the first buyer's checkout payment is completed, then both orders will go through, resulting in a negative inventory quantity remaining in stock (e.g., -1).
We do try to catch the majority of these cases by rechecking available inventory every time a buyer's cart is shown, so if a completed checkout alters your inventory in a way that affects a buyer's in-cart quantity while they are still browsing your site, their order quantity will automatically be adjusted to match available inventory the next time they view their cart before checkout. The only more foolproof way around this would be if we adjusted stock based on what quantity buyers have merely added to their cart even before they go to checkout, but that would simply tie up all your available inventory in pending cart orders which may never proceed to or complete checkout at all.
