This occured at work today where we had upgraded Rails to 2.3.5 on one of our client sites and the contact form that had been written back in the day decided to blow up and throw this lovely error.
undefined method `human_name' for ContactUs:Class
This was occuring since the ContactUs model was not inheriting anything from Rails itself, it was a standalone class. A few quick Google's later and the solution was to put this into the model to replicate the methods that Rails was expecting...