Ran into an interesting bug today and was wondering if anyone else has come across it.
I took a default Magento CE installation (126.96.36.199) and added in the dummy data. Then created a new category called "Software" in the Computer > Electronics category and added a single Downloadable Product to it.
I then attempted to create a price rule where if you added a product that had a category of Furniture (category 22) to the cart along with a downloadable product (category 35), you would get a flat, $15 dollar discount on your order.
Just to start off, I created a coupon that would be applied if any item with category ID of 22 was added to the cart.
This works just fine if you add just, say, an Ottoman to your cart... but if you then also add a downloadable product to your cart, the rule fails to apply.
If I attempt to do anything with price rules when there's a downloadable product in the cart, the rule fails.
Turns out this is simply a little bit of a complex rule. Final settings to accomplish what I wanted looked like this...