Found this problem while experimenting with the flx package
( https://github.com/lewang/flx ).

Put the two attached files somewhere on you load-path.

emacs -Q

eval this:

(require 'flx-ido)
(ido-mode 1)
(setq ido-enable-flex-matching t)
(setq flx-ido-use t)


Now do C-x b for switching to another buffer. Once the list of
candidates is shown, press C-s (which would usually cycle to the next
candidate.) Emacs will start using 100% CPU and consuming memory at a
very quick rate. C-g makes Emacs responsive again.

flx developer reported that the bug is not reproducible on his Emacs
24.3.1.

The original bug report is here:

https://github.com/lewang/flx/issues/6


In GNU Emacs 24.3.50.5 (x86_64-unknown-linux-gnu, X toolkit)
 of 2013-04-06 on qcore
Windowing system distributor `The X.Org Foundation', version 11.0.11303000
System Description:	Ubuntu 13.04

Configured using:
 `configure --with-x-toolkit=lucid --without-toolkit-scroll-bars'

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent input:
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <menu-bar> <help-menu> <send-emacs-bug
-report>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Starting new Ispell process [/usr/bin/aspell::default] ...
Checking spelling of INDEFINITITELY...
byte-code: Window #<window 0x10edd98 on  *Minibuf-1*> too small for splitting

Load-path shadows:
None found.

Features:
(shadow sort gnus-util mail-extr ispell emacsbug message format-spec
rfc822 mml easymenu mml-sec mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils time-date tooltip ediff-hook
vc-hooks lisp-float-type mwheel x-win x-dnd tool-bar dnd fontset image
regexp-opt fringe tabulated-list newcomment lisp-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core frame cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese hebrew greek
romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process inotify dynamic-setting
font-render-setting x-toolkit x multi-tty emacs)