Traits defining equals, combined with case classes

I just contributed a "common programming mistake for Scala developers", in the relevant thread over at stackoverflow (which seems to be turning into a mania, given I contributed quite valuable time myself without really thinking about it). It describes a pitfall from the interplay of case classes, traits and equality defined in the latter - by no means this can be really a "common" mistake, but it is not obvious enough so some help to keep it uncommon doesn't hurt.