Add to Cart Buttons Maintaining Multiple Carts

[insiteadvice]

I'm having a problem with Add to Cart buttons. Let's say I have 4 products (A, B, C, and D), and I do the following:

1. I click A's Add to Cart button. A new tab (from now on referred to as "cart tab") opens displaying my cart with one A, just like it should.
2. I click B's Add to Cart button. The cart tab refreshes and A is gone, but B is in there.
3. I click C's Add to Cart button. The cart tab refreshes and B is gone, but A and C are in there.
4. I click D's Add to Cart button. The cart tab refreshes and A and C are gone, but B and D are in there.

There are over 30 products (most with options), and I haven't checked all of them, so I don't know if there are more than two carts maintained.

I compared the code for the Add to Cart buttons for a handful of the items; aside from names and prices (and sometimes options) the only difference is the value of the value attribute for the hosted_button_id:

<input name="hosted_button_id" type="hidden" value="1234XXX" />