Securing the students view
When I told a colleague to look again at the work of a particular student they went to the students view tab and typed the name in the left hand box to look for the student. This way they matched the exam of the student with copy # 1 to other students. As a result, all grading done for those students was merged as well. I could un-merge the copies by re-assigning copy # 1 to the correct student. However the grading remained merged. Fortunately I already exported a copy of the detailed results, so I can manually re-apply the grading for all students involved if necessary. Unfortunately it's not the first time this issue has happened.
Considering the grading in zesje is often at least partially done by people who do not understand zesje completely as they only need to grade (not do the other jobs involved) and the ease with which you can create havoc by messing around in the student view tab; it would be good if this was somehow made impossible/harder.
There are many different ways to approach this of course; from different roles for different people, to a "lock the connections made" button that has to be pressed again to make changes, to even just a pop-up box explaining what's going to happen if you re-assign the work of some student to another student after some grading has been done, asking for confirmation.