I ran into this error the other day when running "cap -T"
/System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/lib/ruby/1.8/rubygems/custom_require.rb:31:in `gem_original_require': no such file to load -- capistrano/ext/multistage (LoadError)
So to fix this l did the following
sudo gem uninstall capistrano sudo rm -rf /usr/bin/cap sudo gem install capistrano-ext sudo gem install capistrano
Needed to do "sudo rm -rf /usr/bin/cap" as doing just the uninstall of capistrano didn't remove all the files l needed it to.