Original HTTP White Paper

What I have always loved about the original spec is (https://tools.ietf.org/html/rfc2616):

  10.4.3 402 Payment Required
  This code is reserved for future use.

Reminds that you that the idea, and the problem, of accepting payments on the web was right there at the start.

cf. Andreessen: "We should have built payments in the browser."(http://mashable.com/2012/05/01/marc-andreessen-browser-payme...)