Points to remember while adding bundler to Rails < 3.x.x applications- When I had modified an application based rails 2.3.4 for bundler, it has taken my time to make it work as i forgot below points.
1. Please check if someone had made any changes inside plugins/gems which you have mentioned in gem file now.
2. Please note of gems if you are mentioning them :groups as they are not available to other groups. eg. Gems mentioned in :groups => :test are not available to :group => :development. Eg: if you run rake tasks which uses gem libraries in development and gems are mentioned in :tests group, it will give you errors for required gems.
3. Also make sure about gems and plugins versions which are being used in app and you mentioned Gemfile.
I hope it might save your time.