Put the following Emacs Lisp into a file named repro.el:

    (get-buffer-create "repro")
    (fundamental-mode)
    (erase-buffer)
    (set-variable 'show-trailing-whitespace t)
    (dotimes (_ 3)
      (insert "x\t\n\f\t\n"))
    (message "%s" (substitute-command-keys "Now type
\\[delete-trailing-whitespace]"))

Now invoke emacs with `-Q --load repro.el`.  You'll see a buffer with a
few lines, each with a red smear at the end indicating trailing
whitespace (the TAB character).

Now, like the echo area is telling you, type M-x
delete-trailing-whitespace RET.  You'll see the red go away on half the
lines (the ones with an "x") but remain on the lines with form feeds.

It's not entirely clear if this is a bug at all, since the doc for
delete-trailing-whitespace doesn't specifically mention this situation;
but it does say that that function considers form feeds to be _not_
whitespace, and also that the function deletes whitespace at the ends of
lines, so it seems it ought to have deleted _all_ the tabs in this case.




In GNU Emacs 25.3.1 (x86_64-apple-darwin13.4.0, NS appkit-1265.21 Version
10.9.5 (Build 13F1911))
 of 2017-09-12 built on builder10-9.local
Windowing system distributor 'Apple', version 10.3.1404
Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  global-magit-file-mode: t
  magit-auto-revert-mode: t
  global-git-commit-mode: t
  async-bytecomp-package-mode: t
  csv-field-index-mode: t
  pyvenv-mode: t
  projectile-mode: t
  desktop-save-mode: t
  savehist-mode: t
  shell-dirtrack-mode: t
  global-auto-revert-mode: t
  recentf-mode: t
  ido-everywhere: t
  diff-auto-refine-mode: t
  global-hl-line-mode: t
  override-global-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  mouse-wheel-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 messages:
History item: 279 [3 times]
Mark saved where search started
Truncate long lines enabled
Mark saved where search started
Mark set [3 times]
Invoking man open in the background
open man page formatted
Mark saved where search started
Making completion list...
Scanning for dabbrevs...done

Load-path shadows:
/Users/erichan/.emacs.d/elpa/let-alist-1.0.5/let-alist hides
/Applications/Emacs.app/Contents/Resources/lisp/emacs-lisp/let-alist

Features:
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magit-process magit-margin magit-mode git-commit magit-git magit-section
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tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
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georgian utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao
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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 kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
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