gotcha2.rb

class Acme
  attr_accessor :a, :b
  def initialize
    @a=[]
    @b=[]
  end

  def foo!
    a<<5
    self  
  end

  def bar!
    b=[5]
    self
  end
end

a = Acme.new
a.foo!.bar!
puts a.a, a.b

quiz.rb

def foo
  Bar||=5
  puts 5
end

def bar
  puts(a) if (a = 5)
end

def baz
  if a = 5
    puts a
  end
end

def qux
  if (a,b = [1,5])
    puts a
  end
end

foo or bar or baz or qux

more gotchas on http://en.wikipedia.org/wiki/Gotcha_(programming)