|Customer might encounter the following behavior:
- An end user has a browser session with only Visualforce tabs open (*.visual.force.com)
- There are no regular Salesforce Classic UI tabs open (*.salesforce.com) in the browser session
- The Activity Reminder Popup Window does not appear
- If the user opens up a developer console in the web browser, they may see an error like, “XMLHttpRequest cannot load https://ap.salesforce.com/ui/core/activity/ActivityReminderRefreshPage?at=1452174220663. No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'https://c.ap0.visual.force.com' is therefore not allowed access."