Ruby will allow you to do some pretty amazing... errr terrifying things.

1-ruby-why-you.rb

class Foo
  def initialize(default = case $glob
                           when "foo" then (def foobar; "hi!" end)
                           end)
    @default = default
  end
end

Foo.new               #=> #<Foo:0x007f849f37be98 @default=nil>
$glob = 'foo'
Foo.new               #=> #<Foo:0x007f849bbf47e0 @default=:foobar>
Foo.new.methods.first #=> :foobar

2-ohno.rb

class Foo
  attr_accessor :bar,
                :baz,
                
  def initialize(foo)
    @foo = foo
  end
end

Foo.new('my-bar')       #=> ArgumentError: wrong number of arguments (1 for 0) 😕
Foo.new.methods.take(6) #=> [:foobar, :bar, :bar=, :baz, :baz=, :initialize=] 😰
f = Foo.new
f.initialize = 'hello'
f.initialize # => 'hello'

#Thanks to Gavin Stark for pointing this one out.

3-rails_this_is_a_bug.html.erb

<section>
  <%= render partial: 'posts/form', locals: {:@user => current_user, :@post => post} %>   
</section>

<!-- This will re-assign the instance variables @user and @post -->