While fiddling with the WP CLI Tools, I noticed that the WordPress.com server is adding an unusual header to its API responses:
X-hacker: Oh, Awesome: Opossum. Googling around, I found what I should have guessed already. Lots of sites have used silly and unexpected headers for various purposes. Often they are just a form of Easter egg, which is likely what this one is right now. Automattic (makers of WordPress) have used this header in the past, apparently. Several sites document a past value of it as
X-hacker: If you’re reading this, you should visit automattic.com/jobs and apply to join the fun, mention this header.
Two amusing discussions of other found headers are at the Royal Pingdom and nextthing blogs.