I recently installed myStat to keep track of this blog’s rapidly increasing number of readers. Unfortunately, the plugin break the rss feed. The problem is the plugin’s myStat_footer() function, which does not check if the page is a feed before deciding to insert the myStat footer image into the page code.

To fix myStat v2.6, locate line 475 in mystat.php and change the myStat_footer() function

function myStat_footer() {
    global $cmn;
    if($cmn->getParam("myStat_debug")==1){$cmn->setDebug('FOOTER LOAD');};
    echo "<img style=(...);
}

like this

function myStat_footer() {
    global $cmn;
    if($cmn->getParam("myStat_debug")==1){$cmn->setDebug('FOOTER LOAD');};
    if (!is_feed()) {
        echo "<img style=(...);
    }
}

Resources:
http://wordpress.org/support/topic/325426