u most probably are using a 1.6 theme
see diferende of a 1.8 js on headerinclude:
Code:
<script type="text/javascript" src="{$mybb->asset_url}/jscripts/jquery.js?ver=1804"></script>
<script type="text/javascript" src="{$mybb->asset_url}/jscripts/jquery.plugins.min.js?ver=1804"></script>
<script type="text/javascript" src="{$mybb->asset_url}/jscripts/general.js?ver=1804"></script>