Only a minor problem but annoying for novice users.
If in a course with a large number of students put into groups a teacher selects a group with more than 20 students in then an alphabetical list (called an initialsbar I believe) appears for firstname and surname. If on this list they select a particular letter e.g "P" for surname then they only see those surnames beginning with S.
If they now use the drop down list of groups to view another group that has fewer than 20 students they will NOT see the initialsbar BUT the previous selection still holds (in the example above they would still only see participants with surnames beginning with S). As the initialsbar is not visible this causes a lot of confusion. It took me quite a time to work out what the problem was as I was usually simply told by teachers that the students had simply "disappeared" from their lists. When I went to investigate they had invariably logged out and ha to log in again to show me the problem and of course the students had magically reappeared by then.
I would suggest the following change in file
change line 389 from
$table->initialbars($totalcount > $perpage);
This will then always show the init1alsbar