I'm a Python/Django guy so I'm not intimately familiar, but is that really the case? I was under the assumption that the handler wouldn't pass any POST vars not in attr_accessible to the model. In which case additional, unexpected ones would become silently dropped.

According to http://guides.rubyonrails.org/security.html#countermeasures you're entirely correct that it won't raise any error, although the keys won't be dropped: attr_accessible drops the attributes when mass-assigned to a model, they're still available in the params hash.