Login to Admin UI as admin, go to the REST docs page for /recordings/calendars e.g.
Complete sample form filling in agentid and click Submit
Results are shown.
Click Submit again. UI shows "200 (OK)" response which is incorrect because there is actually a 304 not-modified response. Old results are shown.
For this particular endpoint it would better not to send etag or any other info so the result is always a new calendar, but in general also important not to show misleading results.