Trying to merge a blocked user into my "spammer" user, MediaWiki displays a "database error" message, and the error_log contains:
PHP Notice: Uncommitted DB writes (transaction from DatabaseBase::query (UserMerge::mergeUser)). in .../includes/db/Database.php on line 4065, referer: http://.../index.php?title=Spezial:Benutzerkonten_vereinigen&newuser=spammer&deleteuser=1
Workaround: un-block the user you want to merge/delete first.
Note: bug 39470 is probably a different issue - at least it has a different error message ;-)