I've created a patch for an issue I ran into where model callbacks will be called multiple time. See ticket: http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/764-observers-on-a-model-are-called-multiple-times
This happens because the class file is required multiple times, and the callbacks are added on every read. The patch adds a check before defining a callback to see if the method name is already added.
