That anything following the main page column tags are preceded by a <br style="clear:both;" />
It may be due to other MediaWiki messages, but those listed above are the most likely candidates. Advanced users may also want to try checking the HTML source code, as the problem may also become apparent there. Probably the easiest browser to do this in is Google Chrome, as it contain a feature called "Inspector"; here, it pairs up the tags for you, allowing you to see incorrectly placed tag with more ease.
How to prevent and fix
Make sure you write accurate code! Always check that you close any tables or divs (and other elements such as main page column tags) that you open.