I think we may have determined what was (possibly) the cause to this. It seems that if there is whitespace at the top of the document Google
sometimes will not add the <!DOCTYPE html> to the top of the page for an unknown reason, and sometimes it will. I have noticed though, that when it does add that doctype to the top of the page even when there is whitespace in the document it strips that whitespace out (you can see it by viewing page source on the currently cached version of your site and compare it to the live site), but when it doesn't add the <!DOCTYPE html> to the top the whitespace is still visible. Only reason I can think of for this happening is that Google has multiple servers that serve the cached pages and they are configured differently on how they handle the whitespace. Why? I don't know. There is nothing in the w3 spec that says you cannot have whitespace at the top of the document, and we had it to separate some variable declarations that we use in PAGE_CONTAINER from the actual page content. This has since been removed, and we have put a backup fix in place in case this fails. Either way, after Xenith 1.5.2 is released this issue shouldn't come up again.
You can see this happening on
http://www.manutdtalk.com/ as well, and if you view page source you'll notice that they also have whitespace at the top of the document.
Also, if XenForo didn't use the
Sidebar class, but instead used the
hasSidebar class on the html element on pages with a sidebar this wouldn't happen either way