The MysteryMan avatar is returned when a non-existing user is requested with an empty default option
When an avatar is requested with a malformed default option with no value on a non-existing user the 'mm.png' file is returned.
Example:
curl -i https://avatars.linux-kernel.at/avatar/nobodyasdasdasdasdashdakhdkjhkaa?d=While not technically against the API this is not the behaviour of both Libravatar 0.1 and Gravatar, where the default 'nobody.png' file is used instead. This is mentioned briefly, although I think not very clearly, in the documentation:
d or default parameter defaults to the Libravatar logo
Issue found with my tests script ivatar.t.