Signs replaced by vandals, pranksters, people who want to fix bad design, or for any other reason by a non-DOT entity.
Inspired by today's story about a David Bowie Street sign in Austin, TX: http://keyetv.com/news/local/sxsw-director-claims-responsibility-for-david-bowie-street-sign
ninjasigning thread: https://www.aaroads.com/forum/index.php?topic=9201