My application has 3 shipping methods, the cheapest of which is free. The free shipping option is only available for orders totaling over $100.
As soon as I built it, I thought of a way a user could easily circumvent this. Add items to the cart totaling over $100, then select your free shipping, then edit your cart to be less then $100 and maintain the free shipping.
I fixed my application to not allow this. I started wondering though, what do you call this sort of design flaw? I know you can call it a bug, but is there a better description that I can use so that someone will understand what I am talking about?