I also check the gnus registry package in emacs 24.2, I find that
there have a piece of code 

,-------------------------------------------------------------------------------
| (eval-and-compile
|   (or (ignore-errors (progn
|                        (require 'eieio)
|                        (require 'eieio-base)))
|       ;; gnus-fallback-lib/ from gnus/lisp/gnus-fallback-lib
|       (ignore-errors
|         (let ((load-path (cons (expand-file-name
|                                 "gnus-fallback-lib/eieio"
|                                 (file-name-directory (locate-library "gnus")))
|                                load-path)))
|           (require 'eieio)
|           (require 'eieio-base)))
|       (error
|        "eieio not found in `load-path' or gnus-fallback-lib/ directory.")))
`-------------------------------------------------------------------------------

However, there is nothing related to 'gnus-fallback-lib' by
searching the directory in '/usr/share/emacs/24.2/lisp/gnus/`.

Is it a bug? 

Best regards,

Easior
-----
Ich bin von heute und ehedam, aber etwas ist in mir, das ist vor
morgen und übermorgen und einstmal.

                                     Friedrich Wilhelm Nietzsche