Someone else has already bought it. Putting it in your cart doesn't reserve it. I've had things bought out from under me in the time my browser takes to process the "buy" button request. Generally speaking, if it's a warehouse deal on Saturday morning you have to buy it as soon as you see it show up if it's something you're interested in. It'll be gone in the time it takes to read the description.
I didn't just put it in my cart, I purchased it.
It was no longer available after I purchased it.
The credit card amazon had on file was cancelled due to some unrelated fraud attempt (someone tried to buy a phone in some other state), so amazon sent me an email telling me to load the new card number. At this point it was in my cart and unavailable to anyone else for purchase.
I had the option to load the new CC info, or canx the order. I cancelled and mentioned it above in case anyone here wanted the set (for woodworking or whatever).
It now shows available again.