You probably heard by now that Facebook has offers users a more secure connection to Facebook by enabling HTTPS (secure HTTP). You can make this change on your own Facebook account under your account settings and then Account Security. Many people have already taken advantage of this.
There is one effect of this change that you’ll need to know if you manage a Facebook Page with custom tabs you’ve created. Some custom tabs that use i-frames are not being shown properly for those users who have enabled HTTPS. Here is an example of what the error will look like on your custom tab:
You will need to consult with the vendor who created your custom app for them to allow for both http and https versions of your i-frame custom tab. Facebook has detailed the requirements here.