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. However, it may be possible for two buyers to add a quantity to their respective orders which can be met with available inventory separately, but not both together. Thus, if one of these buyers takes their order to checkout before the other buyer completes their own checkout payment, or if they both go to checkout at the same time, then both orders will go through, causing you to become oversold, resulting in a negative inventory quantity 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, and that would simply tie up all your available inventory in pending orders which may never proceed to checkout at all.
