It's hard to tell where that error is, given IE's spotty debug dialogs, but it might have something to do with step in jQuery 1.3.x:
step: {
opacity: function(fx){
jQuery.attr(fx.elem.style, "opacity", fx.now);
},
_default: function(fx){
if ( fx.elem.style && fx.elem.style[ fx.prop ] != null )
fx.elem.style[ fx.prop ] = fx.now + fx.unit;
else
fx.elem[ fx.prop ] = fx.now;
}
}
Actually, I just tested it in Safari 4beta Mac, and it didn't work at all. No error, no add class.