I use Sitefinity and have been getting this error on some websites including zubairahmed.net.
This error is so weird that it appeared randomly and vanished on subsequent page load. Recently I found something that I believe was causing this error.
It appears that this error is linked to the available memory on the server hosting your website, so as soon as the server reaches its limit it flushes the cache and the web application throws the error, to work around that Sitefinity has put a web.config setting to cache the web pages in Database, surprisingly Sitefinity did not tell anyone about this hidden feature until I googled it (not that I didn’t do it before) and found it here.
So all it required was changing this in web.config
and so far no issues, hope it helps someone out there.