I'm using Drupal 7.22 with GMap 7x-2.7 and Location 7x-3.0-rc1 modules, and Jef Poskanzer's Clusterer in the 'thirdparty' folder.

When displaying node map there is no problem with using no marker manager at all (apart from nodes at same location being immediately one behind the other)

There are problems when I use Clusterer.

Where there are clusters everything work fine, but where there are no clusters instead of the title of the node appearing on hover I am seeing the whole URL as in the attached image.

Clusterer.PopUp = function(cluster){    
    var clusterer = cluster.clusterer;    
    var html = '<table width="300">';
    var n = 0;
    for (var i = 0; i < cluster.markers.length; ++i){
        var marker = cluster.markers[i];
        if (marker != null){
            ++n;
            var icon_marker = marker.getIcon();
            html += '<tr><td>';
            html += '<img src="' + icon_marker.url + '" width="' + (icon_marker.size.width / 2) + '" height="' + (icon_marker.size.height / 2) + '">';

If marker.getIcon() returns undefined, the method will blow up like this:

Uncaught TypeError: Cannot read property 'url' of undefined

This method fires a cluster is being clicked.

See also Clusterer2.js: marker.getIcon() returns null, and Clusterer.PopUp() blows up. in the drupal.org GMap issue queue.
I expect this is not an either-or, but both the Drupal module and the clusterer should add some sanity checks for early problem detection.

Btw, I remember having used the clusterer successfully in past projects, and having been happy with it. So something must have changed in the clusterer or in the Drupal module, or my setup is not correct. But even if my setup is not correct, adding some sanity checks can't be wrong.

Thanks!