...Ah-hah, and scrolling up I see to check my JavaScript console in Chrome, which offers this error:

function _initUserhead(context) {
    $("span.ljuser",context).each(function() {
        var $usertag = $(this);

        $("img", $usertag).each(function() {
            // if the parent (a tag with link to userinfo) has userid in its URL, then
            // this is an openid user icon and we should use the userid
            var $head = $(this);
            var $parent = $head.parent("a[href]");
            var data = {};
            var userid;
            if (userid = $parent.attr("href").match(/\?userid=(\d+)/i))

s.dreamwidth.org 379 Uncaught TypeError: Cannot call method 'match' of undefined

                data.userid = userid[1];
            else
                data.username = $usertag.attr("lj:user");
            if ( !data.username && !data.userid ) return;

            data.type = "user";
            $head.contextualhover( data );
        });
    });
}

That error doesn't show when I filter out the entry causing the problem.