The origin of this post is a tweet of PragDav Retweetted by Igrigorik (http://twitter.com/igrigorik/status/22767684142)
I've digged a bit to answer why :_:: == :_ is evaluated to true.
First, you can access to instance or class methods in ruby like this : Foo.new::say_hello => "Hello"
:_ is a symbol so an instance of Symbol class
So
:_ has methods(you can list it by :_::methods)
Therefore
:_:: == is the call to the method == of the instance :_ of the Symbol class
Therefore
:_::==:_ can be interpreted as == (:_, :_) where == is the method
As == (:_, :_) is true , :_::==:_ is true
QED
