One of my teachers reported what I think is a bug when using activity completion. One of her students pointed out a typo in the forum introduction. The forum gets marked complete when students post one discussion and one reply. When she went to fix the typo, the changes would not save unless she removed the activity completion setting completely. She did not make any changes to the activity completion settings. I don't think this should happen as she was not making any changes there.
I have duplicated this with glossary and OU blog on 2.3.3+, and on demo.moodle.net using 2.4.1+. This occurs when requiring a certain number of posts. No changes can be made to any settings without removing activity completion requirements. I was not even able to change the max attachment size. It does not happen when allowing a student to manually mark an item complete or requiring a student to view to mark it complete
I searched for a report of this, but did not find anything. I will report to the tracker unless a wiser Moodle can provide a logical reason for this behavior.