If an assignment is set up for group submission and a student is in more than one group, their submission is made to the default group. This is extremely counterintuitive behaviour (if not an outright bug) and is not documented in the assignment settings documentation (http://docs.moodle.org/26/en/Assignment_settings#Groups_submission_settings)

I do understand that having students in multiple groups for group work doesn't make a lot of sense, and wouldn't necessarily expect it to be supported (particularly given the complications in producing a single grade for the student in the assignment), but the behaviour feels very "undefined" at the moment. I think it would be preferable to have it explicitly "unsupported", perhaps with the student being unable to submit and seeing a clear message telling them that they need to get their group membership sorted out before being able to submit.

To reproduce:

1. Create two groups, and add the same student to both.
2. Add some more students to the course (don't add to any groups)
3. Set up an assignment with group submission
4. Log in as the student in the two groups and make a submission
5. Log in as one of the other students and confirm that you see the submission as the default group submission.