Don't use product thank-you URL if quantity is 0
The cart may be doing what you expect. But it's not doing what WE expect.
We have a standard bundle set up, where ordering one product causes several other products to be put in the cart. The way we set things up, we put one (1) copy of the main product in the cart, and zero (0) copies of the other products. We let the customer change the zeroes to ones if they want any of the additional products.
This has always worked fine, but we ran into a glitch today.
One of the additional products happened to have a custom Thank-You URL link defined. Few of our products have a custom Thank-You URL, and it is the first time we have included one as an additional product in a bundle.
Someone ordered the main product, but left the quantities of the additional products at zero. When they submitted the order, they were taken to the Thank-You URL page for the additional product they did not order.
Now, I can see that for a lot of bundles, people probably don't put additional products in the cart with quantity zero. So when customers purchase the bundle, they purchase all of the products in the bundle, and if one has a Thank-You URL it is appropriate to go to it.
But in our case, the fact that the quantity in the cart of the product ordered is zero means that the customer did NOT order the product, and so should not have been taken to the Thank-You URL.
So I think the program logic ought to check not just for the EXISTENCE of a Thank-You URL for a product in the cart, but also that the QUANTITY of that product in the cart be non-zero. And if it is zero, don't link to the Thank-You URL.