I kept getting an odd error with Nokogiri on my new Macbook:
WARNING: Nokogiri was built against LibXML version 2.9.0, but has dynamically loaded 2.7.8
With help from this post I was able to resolve the issue. It seems that "This happens because the Lion system default libxml2 (loaded at bootstrap) is used, regardless of which libxml2 Nokogiri was built against". No real original work on my part here other than a Google search. However I wanted to spread the effort of Michele Gerarduzzi a litte further and give credit where credit is due.
The relevant post: Get rid of Nokogiri LibXML warning on OSX Lion