400 vs 500

400 vs 500 explanation

Controller action expecting certain input.

In case the input is not provided, the first version will fail with a 500 exception, 
a stack trace and an airbrake notification.
But this is just invalid input and not an error for us to care about.

Instead, like the second version, it should just return nothing and a status code 
400 (Bad Request), effectively telling the user agent, that 

1) This is your fault, your URL / input data is wrong
2) This request will always fail - do not retry! (as opposed to a 500, that may work later)

Also, this is faster and does not spam our Airbrake.

picky_action

def do_something
  if exp = params[:expected]
    some_function
  end
end

safe_picky_action

def do_something
  if exp = params[:expected]
    some_function
  else
    error_400
  end
end