lolruby.rb

class Client
  attr_reader :key,

  def initialize(key)
    @key = key
  end
end

Client.new("moo") # results in: ArgumentError: wrong number of arguments (given 1, expected 0)

Can you figure it out? Expand for spoiler!

There is a trailing comma on the attr_reader. This causes the parser to see this as:

class Client
  attr_reader :key, (
    def initialize(key)
      @key = key
    end
  )
end

Since method definitions return their own name, this is effectively the same as:

class Client
  def initialize(key)
    @key = key
  end
  attr_reader :key, :initialize
end

So the initialize function gets overwritten with a reader for he @initialize instance variable.