1) Run HelloDragAndDropSwing
2) Drag 'DRAG ME' to 'DROP HERE'
3) When you drop, you get the following X error:

The program '<unknown>' received an X Window System error.
This probably reflects a bug in the program.
The error was 'BadWindow (invalid Window parameter)'.
  (Details: serial 549 error_code 3 request_code 2 minor_code 0)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the --sync command line
   option to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)