It's the anti-spam feature Aksimet. It will delete any post it thinks is spam, and if that post happens to be the first one the entire thread disappears. Unlike subsequent posts that are simply marked as deleted and easily restored, finding the OP to undelete it can be a challenge. (Not a problem for Jedi's and Wookies, however.)
Best way to avoid this from happening is to refrain from editing your thread-starting post. Aksimet seems to be more finicky when evaluating a just-edited post. So when possible, it's better to append a correction as a new post rather than editing the top post. Even at the risk of making yourself seem a little less erudite.