Wikitext is hard (no citation needed) and even the most experienced of us make rookie mistakes with it sometimes. Sites generally speaking encourage the use of the preview button...except in WikiForum's case the only available button is "Save reply"! (As seen on this forum thread and basically any forum thread once you're logged in.)
LQT has these buttons (well, "Show preview" as a button and "Cancel" as a link); see TWN's Support page (while logged in as a user w/ sufficient edit rights) for a live example of that. Maybe we could lift some code there?