Test credit card numbers able to make purchases

Hi,
I'm curious, what is to prevent someone from using one of the test credit card numbers to place an order. I mean if I test my merchant account module by setting up a purchase and then running it through the module, the system says the sale was approved, the item is removed from inventory and I now have an order that is supposed to ship.

What is to prevent someone else using that number to place an order themselves?

Where, other than my merchant account statement will I know that no money came in?

Does that number work with your merchant account? I think it doesn't work if you're in production mode, or not in test mode.

JP

The number is processed and returned as a successful order, I am assuming by my merchant account. Quantities are removed from my inventory. The payment does not show on my m.a. activity screen.

The shop is live. For now I've turned off my m.a. and only accept payments through paypal until this is resolved.