I went to test the URI class from the Addressable gem, looking for a replacement for the URI of the standard ruby library.  The gem installation went smoothly, but as soon as I tried to use it, I got this error:

irb(main):003:0> require 'addressable/uri'
LoadError: no such file to load -- addressable/uri
 from (irb):3:in `require'
 from (irb):3

Googling the main error (“LoadError: no such file to load”) turned up this blog post by bparanj. While it talks about Selenium, the solution applies to any gem.