I am so sad you werent able to get your bag on discount!
I would say you just had bad luck. I've never seen the website play games like that. If Kate Spade didnt want to sell the bag at the discount, then it would have never been marked on sale in the first place. I have seen items go in and out of stock multiple times over the life of an item. I feel KS must be pretty diligent with getting returns relisted as in stock. I always leave out of stock items in my cart to see if they return. I even followed an item through this last sale watching it go out of stock and back in stock and out again before deciding to stop following because i didnt want it.
Most likely, other people were waiting for a % off sale and were just lucky to find the sale as soon as it started, the bag sold out, and then later someone(s) returned the bag.
I agree with reginaphalange about calling a store. KS sales people have always been extremely friendly and helpful. Most of KS sales are both in store and website. This last 30% off sale was in stores and website. Sometimes in store will have a slightly better sale that isnt advertised online or by email.