From Facebook Developers Wiki

Re the comment "You can only use this once on a profile" - I find that untrue; I use it multiple times and it works fine. Anyone else agree?

Yeah I seem to be able to use this tag several times on a profile. Not sure what the correct behavior is though.

This doesn't seem to work for me inside an <fb:tabs> within a canvas. Allowing an <fb:tab_item> to be visible or not would be really helpful (e.g. do I show the 'Invite' tab or 'Settings' tab or not).

fb:visible-to-owner doesn't seem to work for actions. Is this a bug?

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I second - Fb:visible-to-user doesnt work for wallpost-action. anyone found a solution or work-around for this ?

[edit] Bugs filed

Thanks for filing bug 1493, Ori -- http://bugs.developers.facebook.com/show_bug.cgi?id=1493

It could be related to bug 1042 - http://bugs.developers.facebook.com/show_bug.cgi?id=1042.

[edit] Comments

Why display a huge white square here, instead of... nothing? I don't get it.

In what situation does a blank white box look aesthetically pleasing?