This was revealed by MDLQA-1196. It's possible for teachers and students to add new topics to a simple forum discussion by manually going to the appropriate URL (your.moodle.instance/mod/forum/post.php?forum=FORUMID). The forum page itself will still only show the first forum posting since it uses get_record() to pull the discussions, but if you have debugging turned on you'll get a db error since multiple records were found:

Error: mdb->get_record() found more than one record!
line 1302 of /lib/dml/moodle_database.php: call to debugging()
line 1262 of /lib/dml/moodle_database.php: call to moodle_database->get_record_sql()
line 1242 of /lib/dml/moodle_database.php: call to moodle_database->get_record_select()
line 144 of /mod/forum/view.php: call to moodle_database->get_record()

The forum shouldn't rely on the interface alone to control this behavior.