When you create a Facebook Developers account and a new iframe application for a custom tab, you are the admin of that application. If the application is for your own page, this isn’t an issue. But, what if you are creating the application for a client? Do they want you to be the admin / owner of the application once the work is complete? Maybe. But leaving control of the application to an outside consultant can potentially lead to problems later.
There are currently two bugs that are affecting the effectiveness of custom page tabs created with an iframe application.
Facebook says they are being worked on and a number of people have commented on the bugs and voted them up so hopefully they will be resolved soon.
But, if you are starting to create custom tabs with iframes or are considering converting your existing tabs, you should be aware of them.