I'm using Magento 220.127.116.11 and have configured my store not to allow backorders, but for some reason some products are ordered having the stock at 0.
I'm using Paypal for the payment process.
Catalog -> Inventory Settings:
Decrease Stock When Order is Placed => Yes
Set Items' Status to be In Stock When Order is Cancelled => Yes
Display Out of Stock Products => Yes
Only X left Threshold => 0
Manage Stock => Yes
Backorders => No Backorders