It can be important to cancel the meeting using the same app you created the meeting in. For instance if you scheduled the meeting in MS Outlook using either the Webex Scheduler or the @webex setting, you need to cancel it from Outlook as well. If you cancel it from the web portal, it doesn't "clear" all the settings in MS Outlook so it will be removed from your account but can remain there for other users. Unfortunately once you've cancelled it from your account, you can't go back and open it to cancel it using the correct app.
