Sites can be programmed to prevent them from being framed with the IFrame module. If these sites are saved as the Source in an IFrame module, the site will not appear inside the IFrame module on the page. Instead, when a user clicks on the page containing this IFrame, they will be redirected to the IFramed website.
To avoid to this problem, do not frame any sites without first seeking the permission of the site owner and confirming with their webmaster that the Source to be framed permits framing. However it is useful to create a test page where to test the IFrame module.