What it says on the tin.
Steps to reproduce:

1. Create a ShoutWiki account if you don't have one already
2. Log into your ShoutWiki account
3. Go to Special:GlobalPreferences
4. Change any option -- skin or language is likely the easiest since that change is super easy to notice
5. Press the save button
6. Observe the cute mw.notify flyout message telling that "Your preferences have been saved"...when actually they have not and you're still seeing the same skin as before.

Pressing the save button causes no entries to be added to the global_preferences table for whatever reason(s).

I also found these in the PHP error log, although these had been logged before I tested out this bug:

Notice: Undefined property: User::$mGlobalPrefs in /var/www/live/w/extensions/GlobalPreferences/GlobalPreferences.hooks.php on line 192
Warning: Invalid argument supplied for foreach() in /var/www/live/w/extensions/GlobalPreferences/GlobalPreferences.hooks.php on line 208