If you are using RVM and trying to code quick ruby scripts you will soon figure out that #!/usr/bin/ruby is going to fail on you if you are not lucky. This is because your ruby is located somewhere under your rvm folder and not in the /usr/bin/ folder. Here is the quick fix to point to the correct version/location of ruby:

#!/usr/bin/env ruby

rest of you ruby code ...

What this does is that it runs the /usr/bin/env to get your ENVIRONMENT info and run the version of the ruby set in your PATH. rvm automatically sets the paths in your ENVIRONMENT to the ruby executable.