Thus far we've been creating user accounts during anonymous checkouts using the generic Rules actions for creating and saving entities. However, this results in unexpected Rules evaluation errors for people who have added custom fields to their user accounts. If any of these fields are required, the user account won't be saved during anonymous checkout completion.

After some pretty rude feedback on an old support request about this precise issue, I did come up with an idea later in the day about how to resolve this. We could simply provide our own action that creates the user account with the bare minimum information, ignoring required account fields (but perhaps either allowing you to set them or simply falling back to the generic form if the user desires). This gets us the continued flexibility of Rules without the liability of some of its more inscrutable functionings. : )