LogManager.loggers is a Hashtable<String,Logger> as of 1.6u14, and the getLoggerNames() method reads as follows:

    public synchronized Enumeration<String> getLoggerNames() {
        return loggers.keys();
    }

If one thread is walking the returned Enumeration object while another thread touches the loggers field (such as by adding a new logger), a ConcurrentModificationException would result.

One way to fix this is to create a copy within the synchronized block, such as:

   return new Vector<String>(loggers.keys()).elements();