defmodule ListHelper do
  def smallest(list) when length(list) > 0 do
    Enum.min(list)
  end
  def smallest(_), do: {:error, :invalid_argument}
end

You might think that calling ListHelper.smallest/1 with anything other than list will raise an error. This however will not happen. If an error is raised from inside the guard, it will not be propagated, and the guard expression will return false. The corresponding clause will not match, but some other might.
So in the example above, if we call ListHelper.smallest(123), we will get the result :something_else, which demonstrates that an error in the guard expression is internally handled.

Interactive Elixir (1.0.2) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> ListHelper.smallest(123)
{:error, :invalid_argument}
iex(2)>