When some XML-related API is used (perhaps, preferences API) on Linux, Mustang b51, it sometimes causes this warning message to be displayed on the screen:

Warning: Could not get charToByteConverterClass!

It was discovered that this message comes from com/sun/org/apache/xml/internal/serializer/Encodings.java,
findCharToByteConverterMethod method.  It contains System.err.println in its exception handler.  The method is very strange as it doesn't return any value except for null.