Cart Selling into the negative quantities

[3x3is9]

Hi,
I have an online store that sells limited edition items. Once gone, there are no more available or obtainable.
Recently, I have had 2 instances where Zen Cart aloud customers to check out when all the items were sold out.
In both cases customers purchased the item shortly after it completely sold out. Once within 1 min, and once within 15 min.
Is there a way to fix this? I had some awkward explaining to do to a disappointed collector, and I could see this happening again.
Is there a patch I can add, or a setting I have wrong?
Thanks!

[chuck]

You need to have your Stock settings (in the Stock menu under Configuration settings) set to disable products after items are sold-out, and to not allow checkout on out-of-stock items.

[3x3is9]

You need to have your Stock settings (in the Stock menu under Configuration settings) set to disable products after items are sold-out, and to not allow checkout on out-of-stock items.

Thank You!