If you tell Dialyzer things that aren’t true, it will believe you 
I suspect fixing the return value to be {:error, any()} | :ok will fix things.
Edit: it also appears that the spec for Memento.Transaction.execute is wrong; it’s listed as {:error, any()} | {:ok, any()} but actually comes from Memento.Mnesia.handle_result, which can only return {:error, any()} | :ok:
