Ox0dea had a bit of fun, and I wanted to know how he did it.

fizzbuzz_ultragolf.rb

# Original from Ox0dea - http://irclog.whitequark.org/ruby/2015-06-29#13259662;
# Yes, it produces FizzBuzz

_=$$/$$;__=_-_;@_=_+_;$_=@_+_;$__=@_+$_;$-_=$__*$_;@__=''<<$-_*($__+$_)+@_;$___=''<<$-_*$__-$__<<$-_*($__+@_)<<@__<<@__;@___=''<<$-_*$__-$_*$_<<$-_*($__+$_)-$_<<@__<<@__;(___=->____{$.+=_;____<<($.%$-_==__ ?$___+@___:$.%$_==__ ?$___:$.%$__==__ ?@___:$.);$.<($__*@_)**@_?___[____]:____})[[]]

# Wtf!? Let's disect it then

_    = $$ / $$  # PID / PID or 1
__   = _ - _    # 1 - 1 or 0
@_   = _ + _    # 1 + 1 or 2
$_   = @_ + _   # 2 + 1 or 3
$__  = @_ + $_  # 2 + 3 or 5
$-_  = $__ * $_ # 5 * 3 or 15

# Now it's getting denser...

@__  =
  '' <<                 # ''
  $-_ * ($__ + $_) + @_ # 122 ((15 * (5 + 3)) + 2) z

$___ =
  ''               << # ''
  $-_ * $__ - $__  << # 70  (15 * 5 - 5)   F
  $-_ * ($__ + @_) << # 115 (15 * (5 + 2)) i
  @__              << # 122                z
  @__                 # 122                z

@___ =
  ''                    << # ''
  $-_ * $__ - $_ * $_   << # 66  (15 * 5 - 3 * 3)   B
  $-_ * ($__ + $_) - $_ << # 117 (15 * (5 + 3) - 3) u
  @__                   << # 122                    z
  @__                      # 122                    z

(___ = -> ____ {
  $. += _ # n - ARGF.lineno - which happens to nicely start at 0

  ____ << (
    $. % $-_ == __ ?         # if mod 15 == 0
      $___ + @___ :          #   'FizzBuzz'
        $. % $_ ==__ ?       # else if n mod 3 == 0
          $___ :             #   'Fizz'
            $. % $__ == __ ? # else if n mod 5 == 0
              @___ :         #   'Buzz'
              $.             # else n
  )
  
  $. < ($__ * @_)**@_ ? # if n < (5 * 2)**2 (100)
    ___[____] :         #   recall the lambda
    ____                # else return the collection
})[[]] # Now call it with an empty array as an accumulator