The 3.0 and lower servers have a command monitoring test which issues a find-getmore-killcursors combination.

The cursorsUnknown field of the reply isn't asserted by the test runner.

Test in question: find.yml in command_monitoring.

      -
        command_succeeded_event:
          reply:
            ok: 1.0
            cursorsUnknown:
              - { $numberLong: "42" }
          command_name: "killCursors"

Changing the contents under cursorsUnknown doesn't fail the test.