When putting an odd number of // in the wiki text, the renderer leaves the last <em>-Tag open and thus produces invalid XHTML which can corrupt the site layout with certain templates. You need no more than "//" as wiki text (but you can put as many text around it as you want) for reproducing that bug, I've put an example on
http://www.dokuwiki.org/wiki:playground. A short test with the last stable releases shows that this bug has been introduced somewhere between 2008-05-05 and 2009-02-14.