I know a forum member who felt very stupid when this happened to him. And he's sitting in my chair right now! I learned my lesson since then and avoid hitting the wrong key when I'm starting a thread.
There was a change in one verison of vBulletin, which we were never able to identify, that made the return-key problem start happening frequently when people create threads. It still happens to people regularly.
Until/unless we find something to do about it, please report the premature posts you notice. The moderators routinely remove them (along with the snide remarks some people post) rather than Wasteland them to avoid embarrassing the poster, who almost always gets his or her thread created properly after another try or two.