Scary awesome.

tapper.rb

class Tapper
  attr_reader :eavesdroppers

  def initialize(source, *eavesdroppers)
    @source = source
    @eavesdroppers = eavesdroppers
  end

  def method_missing(method, *args)
    if @source.respond_to?(method)
      valid_eavesdroppers = @eavesdroppers.find_all {|t| t.respond_to?(method) } 
      define_singleton_method(method) do |*args|
        source_return = @source.send(method, *args)
        begin
          valid_eavesdroppers.each {|t| t.send(method, *args) }
        rescue
          true # swallow errors from eavesdroppers
        end
        return source_return
      end
      send(method, *args)
    end
  end

  def eavesdropper
    @eavesdroppers.first
  end
end