It's a government censorship-related error honoring the late Ray Bradbury.
Googler Wants Error 451 for Government Censorship : Read more
Googler Wants Error 451 for Government Censorship : Read more
There may be error codes of 451 for other things, but there is currently no error 451 in HTTP, which is the type of error code a web page would produce. The 400s go up to 417, and then skip up to 500. So, we're in the clear.