I dunno...

code_that_apparently_should_work.rb

# http://stackoverflow.com/a/19323467/320438

def plusone(x)
  x + 1
end

[1,2,3].map(&:plusone)

irb.rb

~$ ruby -v                                                                                                 *[master]
ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]
~$ irb                                                                                                     *[master]
irb(main):001:0> def plusone(x)
irb(main):002:1>   x + 1
irb(main):003:1> end
=> :plusone
irb(main):004:0>
irb(main):005:0* [1,2,3].map(&:plusone)
NoMethodError: private method `plusone' called for 1:Fixnum
	from (irb):5:in `map'
	from (irb):5
	from /Users/tim/.rubies/ruby-2.1.2/bin/irb:11:in `<main>'
irb(main):006:0>

pry.rb

~$ ruby -v                                                                                                 *[master]
ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]
~$ pry                                                                                                     *[master]
[1] pry(main)> def plusone(x)
[1] pry(main)*   x + 1
[1] pry(main)* end
=> :plusone
[2] pry(main)>
[3] pry(main)> [1,2,3].map(&:plusone)
ArgumentError: wrong number of arguments (0 for 1)
from (pry):1:in `plusone'
[4] pry(main)>

class Numeric
  def plusone
    self + 1
  end
end

[1,2,3].map(&:plusone)
#=> [2,3,4]

Sorry guys this is all my fault