I had a page that has had a FB “like” button forever with this code:

<script src="http://connect.facebook.net/en_US/all.js#xfbml=1"></script> 
<fb:like layout="button_count" show_faces="true" width="80" font="verdana"></fb:like>

I just finished using a bit of the new Facebook API to add some Facebook Connect functionality, including a “Login with Facebook” button in the sidebar of all pages in addition to the Facebook JavaScript SDK asynchronous loader:

<script type="text/javascript">// <![CDATA[
  window.fbAsyncInit = function(){
	FB.init({
	  appId   : '...',
	  session : ...,
	  status  : true,
	  cookie  : true,
	  xfbml   : true
	});

	FB.Event.subscribe('auth.login', function(){
	  window.location.reload();
	});
  };
  (function(){
	var e = document.createElement('script');
	e.src = document.location.protocol + '//connect.facebook.net/en_US/all.js';
	e.async = true;
	document.getElementById('fb-root').appendChild(e);
  }());
// ]]></script>

1. Trying to click on the “Login with Facebook” button would do nothing but result in this JS console error: “FB.login() called before calling FB.init().”
2. The page would just keep reloading itself.

You may have guessed just by looking at the above code what the cause of the problem is. The pre-existing “like” button code is loading the all.js script synchronously, but the Facebook Connect code is also loading the same file, but asynchronously. The fix is even more obvious: Remove the old <script> tag that is paired with the like button. Et voila, fin.