Let’s image that I have a behaviour:

defmodule Bylins.Parser.API do
  @behaviour __MODULE__

  @callback get_page(integer()) :: {:ok, String.t()}
end

And I have a GenServer implementing this behaviour:

defmodule Bylins.Parser.ReqAPI do
  alias Bylins.Parser.API
  use GenServer
  @behaviour API
# ...
  @impl true
  def get_page(i) do
    GenServer.call(__MODULE__, {:get_page, i})
  end

  @impl true
  def handle_call({:get_page, i}, _from, req) do
    {:reply, :error, req}
  end
end

This implementation is clearly wrong, because get_page is obliged to return {:ok, String.t() and not :error. But Dialyzer does not catch this!
Although I can manually type the handle_call method, in the future, if the behaviour changes its spec, I can forget to update the spec on handle_call, leading to errors. And there can be usability problems if I add multiple handle_calls to this module - if I understand correctly, I will be forced to cram all of possible options into one spec.

Is there a better way to handle this situation? =-)