This user name is unavailable. It is reserved for use by the software system, when an expected user name is missing. "Unknown user" may appear in page history when importing a file containing a revision-deleted username, or if the import file is damaged. It is the modern designation of corrupted data in some rolled-back edits from May 2004 when the English Wikipedia began using MediaWiki 1.3 (see this diff at Tropical cyclone and bug #789 in the old SourceForge system). It is unclear whether the software uses this user name in any other situations.
Note: This account was briefly used on English Wikipedia on 25 Feb 2004 and on 13 Mar 2004. (Those edits are now attributed to Unknown user~enwiki). It is now on the system list of reserved usernames which cannot be registered or logged in from.
