Because system-classpath uses java.class.path, it doesn't work when running in a classloader other than the application classloader.
This happens when running in any container that uses a custom classloader (e.g. Jetty, Cake, TomCat).
The fix is to use (.getURLs (.getClassLoader clojure.lang.RT)) instead of (System/getProperty "java.class.path"). A patch is attached.