Any of these HTML elements can trigger the warning message. If you are using resources from a third party, you may need to contact them for instructions on using their product on a secured web page.
A frequent cause of this message seems to be embedded Macromedia Flash movies. Simply change the "CODEBASE" attribute in the "OBJECT" tag to refer to "https" to solve this problem. For more information, follow this link: http://kb2.adobe.com/cps/165/tn_16588.html