This command line iex --erl +A 20 -S mix crashes iex with an erl_crash.dump file on Ubuntu Precise with otp_19.1 and Elixir 1.3.4. Any readme’s on where I should start to figure this out? On my macbook this command works just fine.

Please try again with iex --erl "+A 20" to make sure that the “20” is passed through as well. In your example only +A is passed to erl, while iex tries to understand 20 on itself.