How can an item that has "Sold Out" continue to sell?

We use v1.3.9h it is heavily modified and we really don't want to start over with upgrades and such...
http://www.koisale.com/KoiStore/

Every week we set up 5 -10 "Koi fish Specials" in our "Specials Category" all are set up with only 1 in the "Products Category" box as these "specials" are first come first serve...

Yesterday, for the first time, we had 3 specials sell to to multiple buyers... I.E. John Smith bought special-001 & Special-003 these were also bought by another person... Jane Doe was able to buy special-001 & Jake Ryan bought special-003.... after John Smith bought them...

when I checked the admin for the "Specials category" I found that the "quantity" column showed a -1 indicating that 2 of these items sold... Initially there was only 1 item.... How could this happen?

It actually happened twice yesterday... up until then it has been working perfectly for a couple of years.....

We send out an E-Mail blast to our "Constant Contact" opted in list of over 4000 people.... so hundreds click the links to the shopping cart at about the same time... weather this has any bearing... I don't know... just more info to consider

Any help would be greatly appreciated

Steve Jasse Blackwater Creek Koi, inc.

That's most likely what happened - a few people were shopping simultaneously. The product's stock isn't updated when someone adds to cart, but instead when checkout is finished and basically when customer reaches checkout_success page. In the meantime, anyone else can add it to cart and go to checkout.

You could probably add some code to the checkout_confirmation page to check if all products are STILL in stock to prevent ordering after the first customer has already ordered...

WetSteve:

We send out an E-Mail blast to our "Constant Contact" opted in list of over 4000 people.... so hundreds click the links to the shopping cart at about the same time... weather this has any bearing... I don't know... just more info to consider
Steve Jasse Blackwater Creek Koi, inc.
Its possible if they click the confirmation button in the same fraction of a second (but unlikely)

Check that...
admin->configuration->stock->Check stock level is set to** true**

If it is?
then something in your checkout files has been altered to prevent this operating correctly