On some wikis, there are log entries whose authorship information has been lost due to bugs. They have user names associated with them, but no users by that name exist, and they aren't valid IP address names either. Special:Log refuses to show them.
For example, this shows an error: https://en.wikipedia.org/wiki/Special:Log/Unknown_user
…even though there are log entries for this user name, which can be viewed via the API: https://en.wikipedia.org/wiki/Special:ApiSandbox#action=query&format=json&list=logevents&formatversion=2&leuser=Unknown%20userThis issue was originally noticed in T344632 (although that bug should be fixed by fixing the log entries).