Andrew Smart: I was able to replicate your error here according to page height.
Using my apps for pages @ http://www.facebook.com/anotherfeed?sk=app_135669679827333
I used javascript to calculate the height of the inner page for 7 seconds "7000ms" after the page was loaded to avoid errors. At the bottom of the tab of the page tab you will see ...
This page took 1.724007 seconds. W: 520 x H: 800 , where the actual iframe is 5704 pixels high.
<iframe name="app_runner_4e581c03cfc387e71622269" id="app_runner_4e581c03cfc387e71622269" style="width: 520px; height: 5704px; " frameborder="0" src="http://static.ak.facebook.com/platform/page_proxy.php?v=4#app_runner_4e581c03cfc387e71622269"></iframe>
It seems that the page returned 800 pixels as the height, no matter how long the page takes, I believe that this is an error in the page_proxy.php file and the error can be sent to bugzilla.
Code i used for page height:
<div id="pagestats"></div> <script> </script>
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