I just sent Dave this same e-mail... but here is how to replicate the "Free Shipping" bug.

Notes: DNN5, Latest Cart, shipping by quantity ordered chosen.

1) Add an item to the cart.

2) Add another item to the cart

3) Go to the cart, choose shipping method

4) DELETE one of the items from the cart.

5) 0.00 shipping, method still selected.

ALSO

1) Add an item to the cart

2) Choose shipping

3) Delete the item

4) Shipping method is remembered in the cart from there forward, shipping fee is “TBD”, but allows you to check out.

This would explain how this is happening so often on my orders... someone fills their cart, then because QOH is displayed in the cart, says "Oh, there are not enough of those!" Deletes that item... adds a different item, and Voila! Free shipping. :(