This problem has caused quite a bit of inconvenience and confusion for new users. In addition to the examples in the above-linked thread, I also know of the following other instances (found through the edit summary search tool on my username):
Affected accounts range in age from relatively old (Froid, August 2006) to fairly new (Theattackcorgi, February 2013), so it's hard to tell if the bug is still a problem now. However it obviously affects a non-trivial number of users, and it would be nice if there signature preferences could be restored to the default values. Therefore I wasn't sure whether to file this bug under MediaWiki/Preferences or Wikimedia/Site requests.