Manual:$wgDebugComments

Details
If on, some debug items may appear in comments in the HTML output. As of MediaWiki-1.8.2,  writes one HTML comment near the beginning of the page body with the template:
 *  

Debug text passed to  is included in the text of this debug comment unless an optional second parameter (named  ) is passed to   which evaluates as true.

Use on production sites is not recommended. Debug comments reveal information in page views which could potentially expose security risks.