In chapter 9 it is wrongly said that messages can arrive out-of-order.

[edevil]

In chapter 9, in the "Messages Are Asynchronous" paragraph, it is explicitly mentioned that "remember that the order in which a process receives messages may not be the same order in which they were sent". However, this is only true if the sending is done by different processes which is not the case in the example.

In the Erlang manual we can find "If there is a live process and you send it message A and then message B, it's guaranteed that if message B arrived, message A arrived before it".