It seems that there may be a bug with the /cart/add.js endpoint when adding products that:
The issue is that you can add more than the available quantity so long as you provide a new unique line item property with the add to cart request (i.e. create a new line item).
Basically it looks like Shopify is treating stock availability by line item, rather than by product variant id. Anyone else ever encounter this? Is this the expected result from the Shopify servers, or might it be "fixed"?