title: In rails, how can I find out what caused a .save() to fail, other than validation errors?
body: <p>I have an ActiveRecord model which is returning <code>true</code> from <code>valid?</code> (and .errors is empty), but is returning <code>false</code> from <code>save()</code>.  If the model instance is valid, how can I find out what's causing the save to fail?</p>