If you're getting this error and you're sure that you are not hosting your web app on a web farm, then this blog post has most of the information you need.
All that because someone decided it would be best to render some hidden fields at the end of the page rather than the beginning...how lame. Anyway I did not like any of the workarounds listed there, but I did find one which I do not particularly hate:
Adding
<meta http-equiv="Page-Enter" content="RevealTrans(Duration=0,Transition=0)" />
UPDATE
It turns out this only hides the content. The invisible content is still there and can be clicked. So it's not a foolproof solution, but it is an option. I no longer use this method.
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