Coming home tonight, I ended up behind a car with the number plate HTTP 418. I was intrigued by this, so I made a note to check it when I got home.
Just checked it on Wikipedia and the HTTP response code means "I'm a teapot".
This was apparently created in 1998 as an April's Fools Joke. I'm impressed that this is an actual response code, impressed that a sense of humour is still in the Internet despite the corruption by industry, and impressed that someone knew this and got a number plate out of it.