[Spivak]

Example Code:

   users = http.client.get("https://yourapi.com/users")

   # Frobulate each user
   for user in users:
      user = http.client.post(user.frobulate_url)
      # While this is running somewhere on the other side of 
      # the world someone deletes one of the users. Oops.

Assuming, users (i.e developers) will just never mistype a URL so you don't have to give useful feedback is just like being a bad netzen.

[NoGravitas]

That's a fair example. And returning 404 (Not Found) or 410 (Gone) is the most useful kind of feedback. More explanation in the body of the 404 response is helpful too, of course.