This is an error that occurs when two different versions of the model have been loaded. I used to hit it in an older version of Rails 3, since the development environment’s model reloader was slightly glitched. The numbers after the class name refer to different versions of the class.
It stands to reason that this sort of error might come up in development mode, but it shouldn’t in test mode, because, by default, classes are cached. See the config/environments/test.rb file to ensure that cache_classes is set to true.
Also check that you’re on the latest version of Rails, 3.0.7. This may be a bug that has since been fixed. While we’re at it, check that you’re on factory_girl 1.3.3. When using the API totally correctly, which I think you’re doing, the only possibilities left are that something is misconfigured or that it’s a bug in someone else’s code.