Scenario
- Create two Shipping Discount offers with 100% discount that apply to the same method (in our case one of the offers required a coupon code and could not be combined with other offers)
- Use "Apply Offers - Stacked Discounts" workflow step instead of "Apply Offers - Greatest Discount"

The offer without the coupon code will be applied immediately and everything works as it should. Now add the coupon code for the second offer and proceed to checkout (in our case we're using the stock one-page checkout). Once you choose the shipping method that the offers apply to you'll see a negative shipping amount. In other words, the 100% shipping discounts are being combined and are now discounting the order beyond the amount of the shipping method. What should happen, in my opinion, is that the workflow step should ensure that the shipping discount amount never goes negative. I can't really think of a reason why you would ever want this behavior. I added a little code to the workflow to do just that.

Looking at the other code in the workflow step I'm concerned that this same issue could happen with other types of discounts. I realize that using the "Apply Offers - Greatest Discount" workflow step would not have this issue, but I want the stacking behavior.