The situation arose where our Certifier gem would not bundle install
Could not find gem 'rspec-core (= 2.5.1, runtime)' in any of the gem sources listed in your Gemfile.
Our gemserver (non-rubygems) only had
- rspec-core-2.6.4.gem
- rspec-core-2.8.0.gem
Analysis: someone had an old gem installed via rubygems, and didn't run bundle update when building the latest certifier. This used to be solved by maintaining the full-history of dependencies on our gemserver.