Recently the Goople Maps API V3 failed to load the resource "markercluster".

The link "https://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/images/m1.png" is down and isn't display the circle lines colors.

The solution is change the url  of the icons's service.

Change the url "https://google-maps-utility-library-v3.googlecode.com/svn/trunk/markerclusterer/images/m1.png" to "https://cdn.rawgit.com/googlemaps/js-marker-clusterer/gh-pages/images/m" at line "MarkerClusterer.IMAGE_PATH" in markerclusterer.js archieve.

See example:

See below the bug's image  and the display after solution.