I've just updated to OS X Mountain Lion and I'm getting the following when working with rails and terminal.
WARNING: Nokogiri was built against LibXML version 2.8.0, but has dynamically loaded 2.7.8
I've had a look at other answers to a similar question, but they doesn't seem to stop the warning message from appearing.
Putting gem 'nokogiri'
above gem 'pg'
in my Gemfile fixed this for me.
My Gemfile didn't have nokogiri in it, but it was a dependency that was in Gemfile.lock, so I put it in my Gemfile explicitly.