RVM local/server version mismatch
A pivot noted that if you are using RVM in both your development and production environments, and you are using Capistrano for deployment, that you could encounter weird errors at deploy time like:
99: rvm_error: command not found ** [out :: app_user] /home/app_user/.rvm/scripts/rvm: line 121: __rvm_conditionally_add_bin_path: command not found ** [out :: example.com] /home/app_user/.rvm/bin/rvm-shell: line 26: rvm: command not found
These errors occur even after you follow the instructions at http://rvm.beginrescueend.com/integration/capistrano/.
The moral- make sure your RVM version numbers match in development and production.