Currently there are two cases of broken user renaming known on Wikipedia, one on de.wikipedia, and one on en.wikipedia.
In both cases a user was renamed, the old account seems not to exist any longer, but actually there are some contributions left for the old account.
http://en.wikipedia.org/w/index.php?title=User:RS2007&redirect=no (note the missing contributions link) was renamed to "User:RS1900" on 2007-09-02, see http://en.wikipedia.org/w/index.php?title=Special%3ALog&type=renameuser&user=&page=User%3ARS2007
But have a look at their contributions:
There's also a note of the bureaucrat who handled the renaming at http://en.wikipedia.org/wiki/Wikipedia_talk:Changing_username/Archive_2#Broken_rename
http://de.wikipedia.org/wiki/Benutzer:Beblawie was renamed to "Weiße Rose" on 2005-11-18, see http://de.wikipedia.org/w/index.php?title=Spezial:Logbuch&type=renameuser&user=MichaelDiederich&page=&pattern=&limit=500&offset=0 (no direct link possible, but you can search for "Beblawie" there)
Now have a look at their contributions:
http://de.wikipedia.org/wiki/Spezial:Beitr%C3%A4ge/Beblawie - there are a lot of 'deleted contributions' too, see http://de.wikipedia.org/wiki/Spezial:DeletedContributions/Beblawie
I'm unsure if this could really be related, but the two cases only have one thing in common. Both renamed accounts have been blocked afterwards, see
http://de.wikipedia.org/w/index.php?title=Spezial:Logbuch&type=block&page=Benutzer%3AWei%C3%9Fe_Rose (first block: 2005-11-30)
So if possible please merge all contributions (including deleted ones) from the 'old' to the 'new' accounts in both cases, and try to find the reason for failure and fix it :-)
Many thanks in advance.