I recently was added to the Coder group on mediawiki.org (username: tbleher).
http://www.mediawiki.org/wiki/Special:Log/rights correctly says

15:42, 23 January 2011 Reedy (Talk | contribs) changed group membership for User:Tbleher from (none) to Coders ‎ (Has commit access)

RecentChanges shows the same line.

The email notification I received was however entirely different:

Date: Sun, 23 Jan 2011 15:45:17 +0000
From: WikiAdmin <wiki@wikimedia.org>
Subject: MediaWiki page User:Tbleher has been created by Reedy
To: Tbleher <ThomasBleher@gmx.de>
Reply-To: reply@not.possible

Dear Tbleher,

the MediaWiki page User:Tbleher has been created on 23 January 2011 by
Reedy, see http://www.mediawiki.org/wiki/User:Tbleher for the current
version.

This is a new page.

Editor's summary: Has commit access

Contact the editor:
mail: http://www.mediawiki.org/wiki/Special:EmailUser/Reedy
wiki: http://www.mediawiki.org/wiki/User:Reedy

There will be no other notifications in case of further changes unless
you visit this page. You could also reset the notification flags for all
your watched pages on your watchlist.

Your friendly MediaWiki notification system

To change your watchlist settings, visit
http://www.mediawiki.org/wiki/Special:Watchlist/edit

Feedback and further assistance
http://www.mediawiki.org/wiki/Project:Help

My user page is on my watchlist, but it hasn't changed since its creation on
10 January 2008
(see http://www.mediawiki.org/w/index.php?title=User:Tbleher&action=history).

So somehow the email text message got mixed up.

Version: unspecified
Severity: normal