Ruby's rather non-deductive behavior difference between lambdas / procs. (Edited for coherency).

gistfile1.txt

1.9.3p0 :004 > require 'pp'
 => true
1.9.3p0 :002 > p = proc { |a| pp proc_args: a }
 => #<Proc:0x0000010090b838@(irb):2>
1.9.3p0 :010 > p.call(1,2)
{:proc_args=>1}
1.9.3p0 :008 > stabby = ->(a) { pp stabby_args: a }
 => #<Proc:0x00000101099410@(irb):8 (lambda)>
1.9.3p0 :009 > stabby.call(1,2)
ArgumentError: wrong number of arguments (2 for 1)
	from (irb):8:in `block in irb_binding'
	from (irb):9:in `call'
	from (irb):9
	from /Users/cez/.rvm/rubies/ruby-1.9.3-p0/bin/irb:16:in `<main>'
1.9.3p0 :011 >