I think he's referring to the fact that you won't try to run some executable that depends on Ruby 1.9.3 and SomeGem 2.9, but you're on your rbenv that is Ruby 2

I think he's referring to the fact that you won't try to run some executable that depends on Ruby 1.9.3 and SomeGem 2.9, but you're on your rbenv that is Ruby 2.0 and SomeGem 3.0.1. I'm not a Ruby developer, but I know this scenario has caused me considerable trouble just trying to use things built with Ruby.