If you already upgraded your Facebook Page, some FBML Custom Tabs (yes, FBML will eventually be phased out by the end of 2011) were incorrectly resized. This has been recently "fixed" by Facebook. That said, they are saying that *if* you still see a glitch on one of your custom FBML tabs, you need to set the body stylle (css) to "margin:0px;padding:0px"
I have checked about a dozen of my clients' pages (I have over 100 for whom I have built custom FBML tabs)... and so far... the "Facebook Fix" seems to work. That said, here is the blog comment by a Facebook staffer (Product Marketing Manager) Thursday, Feb 10, 2011 at 6:11pm:
FBML page tabs should be 520px. In iframe Page there is a bug that makes iframe 512px by default. You can work around this by setting the style of your body tag to style="margin:0px;padding:0px". Fix coming soon.
