In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2012-09-09 on trouble, modified by Debian

'should' from lisp/emacs-lisp/ert.el fail to create character from number, for
example evaluate one of these expressions:

  (should (equal #x1000000 1))
  (should (equal 1 -1))

Problem come from "?%c" in:

  (defun ert--explain-format-atom (x)
    "Format the atom X for `ert--explain-equal'."
    (typecase x
      (fixnum (list x (format "#x%x" x) (format "?%c" x)))
      (t x)))

Another problem from (format "?%c" x) is performance penalty when "x" is rare
character code (font library intensively scan for missing character glyph
among all system available fonts causing 5 second delay and 100% hard disk
usage).

I recommend remove formatting to character as amount of problems are larger
then amount of benefits.

I discover this issue when start write tests for low level binary parsing
library (ASN.1).