Steps to repeat, eval the following and run `ert'.

(ert-deftest bad-equal ()
  (should (equal 23 -50)))

Instead of a normal Ert error, the following is reported:

A bad-equal
    aborted

When placing the cursor on the error and press "d" (to run the test case in
the debugger), the following call stack is presented:

  apply(debug (error (wrong-type-argument characterp -50)))
  ert--run-test-debugger([cl-struct-ert--test-execution-info...
  #[128 "\301\300\"\207" [[cl-struct-ert--test-execution-info ...
  format("?%c" -50)
  ert--explain-format-atom(-50)
  ert--explain-equal-rec(23 -50)
  ert--explain-equal(23 -50)
  apply(ert--explain-equal (23 -50))
  (list :explanation (apply -explainer- args-1))

Clearly, the code is trying to print -50 as a character.

This worked in Emacs 24.5, where Ert reported:

F bad-equal
    (ert-test-failed
     ((should
       (equal 23 -50))
      :form
      (equal 23 -50)
      :value nil :explanation
      (different-atoms
       (23 "#x17" "?")
       (-50 "#x3fffffffffffffce"))))


Sincerely,
    Anders Lindgren


In GNU Emacs 25.0.50.158 (x86_64-apple-darwin14.5.0, NS appkit-1348.17
Version 10.10.5 (Build 14F27))
 of 2015-10-16
Repository revision: ff4798b8b493ba1ec51dcb1c59a11824865124b8
Windowing system distributor 'Apple', version 10.3.1348
Configured using:
 'configure --with-ns --without-dbus'

Configured features:
ACL ZLIB TOOLKIT_SCROLL_BARS NS

Important settings:
  value of $LC_CTYPE: UTF-8
  locale-coding-system: utf-8-unix

Major mode: ERT-Results

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-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
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
bad-equal
Ran 1 tests, 0 results were as expected, 1 unexpected
Running test bad-equal...
Entering debugger...

Load-path shadows:
None found.

Features:
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mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
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korean japanese eucjp-ms cp51932 hebrew greek romanian slovak czech
european ethiopic indian cyrillic chinese charscript case-table epa-hook
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loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 89635 5963)
 (symbols 48 19436 0)
 (miscs 40 78 159)
 (strings 32 15385 4052)
 (string-bytes 1 463936)
 (vectors 16 12257)
 (vector-slots 8 415700 5035)
 (floats 8 162 183)
 (intervals 56 271 6)
 (buffers 976 14))