otp-21-bug.exs

defmodule A do
  def err(_, <<length, data :: binary>>) do
    case {length, data} do
      {0, _} -> data
      {4, <<rest :: binary>>} -> rest
    end
  end
end

<<0>> = A.err(0, <<0>>)
<<2>> = A.err(0, <<4, 2>>)

defmodule B do
  def err(_, <<length, data :: binary>>) do
    case {length, data} do
      {0, <<rest :: binary>>} -> rest
      {4, _} -> data
    end
  end
end

<<1>> = B.err(0, <<0, 1>>)
<<2>> = B.err(0, <<4, 2>>)