[Carioca]

Probably using a framework (Django, for instance) that doesn't have a default 404 path.

[ragsagar]

If 404 template is absent, in the case of django it will show Not Found The requested URL /whypost was not found on this server.

[davyjones]

Guessing by what curl -I says, looks like lighttpd + webpy.

[GVRV]

BuiltWith shows it's using Rails.