Things that passed my mind when reading the Basics section

Reading the Basics section for the first time, there were some - though few - things I did not understand on a first read, though the rest is really well structured.
The first issue applies to an example for the Model.set() method, were unobvious uses of hasChanged are used, while the topic of the surrounding paragraphs is not fully demonstrated.
The second issue is an enhancement request, discussion validation in the context of initialization. As Jeremy Ashkenas points out, the current behaviour is going to stay: https://github.com/documentcloud/backbone/issues/2628