As a follow-up of MDL-48159, I stumbled upon a minor thing:

If I set the "Require group to make submission" setting in an assignment activity to yes, students who are not in any group are prevented from submitting their assignment and are presented a warning instead that they should contact their teacher. This is what MDL-48159 was about and it's working well.

Now, if there are students who are in more than one group, these users are prevented from submitting their assignment as well because Moodle would not be able to detect the group for which the student is submitting his group assignment. I don't remember it correctly, but I think this behaviour was also discussed when this patch was developed. Anyway, I like this behaviour.

My problem is that the warning string which is presented to the student is only targeted to the fact that the student is in no group, but does not deal with the fact that a user might be in more than one group.