From Jason Beverage, "Here is a fix for a small race condition in osgDB::makeDirectory. It attempts to create all the directories in the given path and stops attempting to make directories when one of them fails. I've added a check to see if the failure occurred b/c the directory was created by another thread or process.

We were running into issues occasionally in osgEarth where multiple threads were writing out files like /1/2/3.jpg and /1/3/4.jpg.  Both threads would try to create the /1 directory and only one of them would succeed.  So the first thread would write out the full /1/2/3.jpg while the second thread wouldn't create the /1/3 directory b/c /1 was already created and the writing of /1/3/4.jpg would fail.
"

src/osgDB/FileUtils.cpp

@@ -214,8 +214,13 @@ bool osgDB::makeDirectory( const std::string &path )
         if( mkdir( dir.c_str(), 0755 )< 0 )
 #endif
         {
-            OSG_DEBUG << "osgDB::makeDirectory(): "  << strerror(errno) << std::endl;
-            return false;
+            // Only return an error if the directory actually doesn't exist.  It's possible that the directory was created
+            // by another thread or process
+            if (!osgDB::fileExists(dir))
+            {
+                OSG_DEBUG << "osgDB::makeDirectory(): "  << strerror(errno) << std::endl;
+                return false;
+            }
         }
         paths.pop();
     }