I've had this issue periodically (~6 times) since last October.
I run several WP blogs and only one has this problem. As luck would have it, it's the only one where I don't have access to the database or server logs. And as luck would doubly have it, that person is away on vacation.
I compose posts in a text editor, including html markup. I copy and paste them into the WP text box, click Schedule or Publish and usually this is fine. I can tell there's going to be a problem if the spinning wheel of death button hangs instead of the page refreshing immediately. (I'm talking about minutes, hours, etc. The problem is not patience.)
I'm only running a few plugins and they're the same plugins as all my other blogs.
Sometimes the post shows up, I test a few browsers to be sure it's there, then come back a few hours later and the post is either gone, leaving only a title, or else it's severely truncated.
Any ideas what's happening and how to make it never happen again?