Funny enough, I thought the exact thing but then I thought it would be a ridiculous reason as it should be handling that the proper way. (It was a quick thought so I didn't consider a few things.) :rofl:
So, I recently hooked up a sub-domain as an add-on domain (with some trickery) and that seemed to have worked (well, still propagating).
However, when I was going to get the new FTP details, I decided to have a look at 'resource usage' as the add-on domain wasn't attached to the account for over...
Anyway, now to (attempt) to answer the question.
As far as I know, it's very resource intensive. They could just move it onto a different server but that's rather a complicated thing to do because it's part of cPanel.
That's why I said "apparently". On the X10 guide, it says that it disables Varnish cache but I do have a doubt that it would disable the whole thing, but yet I thought it could help in some way. :P
If it does occur again, the following string that gets placed in the '.htaccess' apparently disables Varnish Cache. It's worth trying out.
Header set Cache-Control "max-age=0, private, no-cache, no-store, must-revalidate"
As this isn't paid hosting, you will have to do so through your registrar (where you registered your domain).
Who or what said this? Because that's not relevant to the situation and the issue is not there now.
I believe the closest thing you can do is a redirect to X10. I mean the normal domain doesn't even belong here as @bdistler said.
EDIT 1: I took a bit of thought a remembered that someone hooked up tumblr with their sub-domain, so I did a simple Google search and I've found a load of results...
It does look as if it "randomly" (AKA getting unknowingly triggered) is bringing up the error on multiple servers. It's like something in relation to linking all of the servers up is having some issue. I mean who knows what could be causing the issue as of now?