If you've compiled Ruby from source, you might get this error when executing script/console:
/usr/local/lib/ruby/1.8/irb/completion.rb:10:in require': no such file to load -- readline (LoadError)
One way of fixing this is to compile readline, which is distributed along with the Ruby source:
cd /opt/src/ruby-1.8.5-p2/ext/readline ruby extconf.rb make sudo make install
This works even after compiling Ruby, so no need to recompile... If you're wondering what readline is then this quote from the project homepage sums it up in one sentence: "The GNU Readline library provides a set of functions for use by applications that allow users to edit command lines as they are typed in."