Under some conditions, checkout creates extra ship groups in the order.
Steps to reproduce:
- create a couple of products P1 and P2 (just to get clear results).
- create a new order and add 1 P1 and 1 P2
- change quantity P1 to 2 (or both) and Recalculate Order (key step really)
- finish checkout
You will see 2 order items linked with ship group 00001 and another ship group 00002 that should not have been created. You may see in the patch that the code address a ship group using index of the order item and the shopping cart creates new ship group if the index is greater than the number of existing groups.
In one of my projects based on 16.11, I noticed even more serious trouble that takes place if both products are drop-shipping order but don't explain it here because it seems do not appear in the trunk.