Comment 6 for bug 942227

To reproduce this reliably, you can just induce frequent background changes with the following code change in background.vala:

@@ -296,6 +299,14 @@
         active_monitor = monitor;
     }

+ bool regen()
+ {
+ loaders.remove_all ();
+ load_background (null);
+ reload ();
+ return true;
+ }
+
     public override void size_allocate (Gtk.Allocation allocation)
     {
         var resized = allocation.height != get_allocated_height () || allocatio
@@ -309,6 +320,8 @@
             loaders.remove_all ();
             load_background (null);
             reload ();
+
+ Idle.add(regen);
         }
     }

I've got a fix for this, upstream branch coming.