Flag: sometimes_error_codes_are_just_a_teapot
The challenge flavortext says that "HTTP error codes are kinda fun" and links to http://zeromutarts.de:8080/.
If we visit a URI for a non-existent resource, we see a custom 404 error page saying that "414 is so much better...":
The 414 HTTP status code means Request URI too long, so let's visit a URI longer than the server limit:
The custom 414 error page contains the flag.
or, via curl:
$ curl http://zeromutarts.de:8080/`python -c 'print "414"*1000'`
<h1>414 - Request-URI Too Long</h1><br>flag{sometimes_error_codes_are_just_a_teapot}
The flag is thus sometimes_error_codes_are_just_a_teapot.
The teapot is a reference to the April Fools' Day RFC on the Hyper Text Coffee Pot Control Protocol.