It seems to be caused by how fast these vintage pipes they keep offering are selling out.
This time they offered 72 vintage Ropp and Bruyere St. Claude pipes and less than 24 hours later there are only 2 left.
The same thing happened last week and they were all gone before I could blink.
I think that the site has trouble with 72 people ordering at the exact same time.
It is even possible that you could get something into your cart and someone else could buy it before you press go.