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I had the weirdest issue when initializing my compass project for Greenie.

>>> Compass is watching for changes. Press Ctrl-C to Stop.
NoMethodError on line ["402"] of /Library/Ruby/Gems/2.0.0/gems/sass-3.4.8/lib/sass/plugin/compiler.rb: undefined method `start' for #<Thread:0x007f90d1afd098 sleep></Thread:0x007f90d1afd098>
  /Library/Ruby/Gems/2.0.0/gems/sass-3.4.8/lib/sass/plugin/compiler.rb:402:in `map'
  /Library/Ruby/Gems/2.0.0/gems/sass-3.4.8/lib/sass/plugin/compiler.rb:402:in `listen_to'
  /Library/Ruby/Gems/2.0.0/gems/sass-3.4.8/lib/sass/plugin/compiler.rb:338:in `watch'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/sass_compiler.rb:46:in `watch!'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/commands/watch_project.rb:41:in `perform'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/commands/base.rb:18:in `execute'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/commands/project_base.rb:19:in `execute'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/exec/sub_command_ui.rb:43:in `perform!'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/lib/compass/exec/sub_command_ui.rb:15:in `run!'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/bin/compass:30:in `block in <top (required)>'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/bin/compass:44:in `call'
  /Library/Ruby/Gems/2.0.0/gems/compass-1.0.3/bin/compass:44:in `<top (required)>'
  /usr/local/bin/compass:23:in `load'
  /usr/local/bin/compass:23:in `<main>'

I solved it by using a more recent version of Ruby using RVM. RVM is pretty cool because you can switch your Ruby versions very easily. Note that I had to re-install compass afterwards.