I had the rules disappear on me, just, you know, not at all showing up...
But, if you quote the post (or edit it with your awesome non-existent mod powers), you can see the post.
I've also seen it happen to a part of the newbie guide (too lazy to check if it's still gone), so I'm guessing it only happens to long posts, which might just mean the server's too lazy to send relatively big files :S