I may have encountered a bug in the shopping cart.
I am logged in on several machines. I put stuff into my cart from one PC then left it there, and accessed the cart from two of the other PCs.
Then, I went back to the first PC and removed all of the items (from the shopping cart page, which told me I'd made too many requests too quickly). Then I added a new item, which I then bought, leaving my cart empty.
When I came back to another PC, I found that my cart contained stuff - things that I had put in my cart months ago, but removed not much later.
I haven't tried to repeat it, so I don't know if it actually is a bug but it did seem a bit weird!