Demonstration of a weird bug in Ruby 1.9.

example.rb

module MyModule
  def some_method; super; end
end

class MyBaseClass; end

class MySubClass < MyBaseClass;
  include MyModule
end

# To trigger this bug, we must include the module in the base class after
# the module has already been included in the subclass.  If we move this line
# above the subclass declaration, this bug will not occur.
MyBaseClass.send(:include, MyModule)

MySubClass.new.some_method

ruby 1.8 output

➜ ruby --version
ruby 1.8.7 (2011-02-18 patchlevel 334) [i686-darwin10.6.0]
➜ ruby example.rb 
example.rb:2:in `some_method': super: no superclass method `some_method' for #<MySubClass:0x1001bc2d0> (NoMethodError)
	from example.rb:2:in `some_method'
	from example.rb:13

ruby 1.9 output

➜ ruby --version
ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-darwin10.6.0]
➜ ruby example.rb 
example.rb:2: stack level too deep (SystemStackError)

Yuck!