I believe I have found a fix for this:
I did some testing and to my surprise experienced the same error myself on Firefox and Safari for windows.
Somewhere here on SO I found a suggestion to disable GZIP compression. I had both dynamic and static compression installed on the Windows Web Server 2008 R2 (iis7). First I uninstalled static which did not solve the problem. But after uninstalling both I have not seen the error. The error pages were (as far as I can tell) all with AJAX calls returning json involved.
All error pages started with something like this:
HTTP/1.1 200 OK Cache-Control: private Content-Type: text/html;
charset=utf-8 Content-Encoding: gzip Vary: Accept-Encoding Server:
Microsoft-IIS/7.5 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET
Date: Tue, 29 Nov 2011 21:45:06 GMT Content-Length: 7534
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