[thelist] <optgroup> bug?

hey all...

i've been using the <optgroup> tag one on my sites, and i've noticed someth=
ing odd.  every so often, the hierarchical menus will shift.  for example, =
the submenu for topic A will apper on topic B, the submenu for topic C will=
 appear on topic B, and so on.  however, if i look at the html code, everyt=
hing is correct.  has anyone else run into this problem?  admittedly, i've =
only seen this in IE5.x for the mac, so maybe this is a browser bug.  if yo=
u want to see the site in questions, it's at http://www.opuszine.com/.  muc=
h thanks...

jason