This is a bug that happens when trying to navigate between two *Help*
buffers.

==Reproduction instructions==

emacs -Q

Evaluate this code:

(global-set-key (kbd "C-c l") (lambda () (interactive) (insert "This
messes up the help buffer somehow.")))

Then open up a help buffer for that function:

C-h k C-c l

And a help buffer for something else:

C-h k a

Then, in the help buffer, go to the bottom, and try to select the
"[back]" link. I get an error: "user-error: You didn’t specify a function
symbol". If I try again, I get a different error: "apply: Symbol’s
function definition is void: nil". These errors happen whether I've
select the link with pressing Enter on the keyboard or by clicking the
mouse.

==End of instructions==

In GNU Emacs 25.1.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 3.18.9)
 of 2016-07-10 built on zck-laptop
Repository revision: 466ee1b3ea76425d201b5d59950e88251870c836
Windowing system distributor 'The X.Org Foundation', version 11.0.11803000
System Description:    Ubuntu 16.04.1 LTS

Recent messages:
Mark set
(lambda nil (interactive) (insert "This messes up the help buffer
somehow."))
Type C-x 1 to delete the help window.

Mark set
user-error: You didn’t specify a function symbol
apply: Symbol’s function definition is void: nil
scroll-up-command: End of buffer [2 times]
apply: Symbol’s function definition is void: nil [3 times]
p is undefined

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11

Important settings:
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

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
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message puny seq byte-opt gv bytecomp
byte-compile cconv dired dired-loaddefs format-spec rfc822 mml mml-sec
password-cache epa derived epg epg-config gnus-util rmail rmail-loaddefs
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr
mail-utils cl-extra help-fns radix-tree help-mode easymenu edmacro
kmacro cl-loaddefs pcase cl-lib time-date mule-util tooltip eldoc
electric uniquify ediff-hook vc-hooks lisp-float-type mwheel term/x-win
x-win term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list newcomment elisp-mode lisp-mode prog-mode register page
menu-bar rfn-eshadow timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese charscript case-table epa-hook
jka-cmpr-hook help simple abbrev obarray minibuffer cl-preloaded nadvice
loaddefs button faces cus-face macroexp files text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote dbusbind inotify dynamic-setting
system-font-setting font-render-setting move-toolbar gtk x-toolkit x
multi-tty make-network-process emacs)

Memory information:
((conses 16 98309 6988)
 (symbols 48 20970 0)
 (miscs 40 262 168)
 (strings 32 18601 4494)
 (string-bytes 1 595182)
 (vectors 16 13861)
 (vector-slots 8 449156 6768)
 (floats 8 188 43)
 (intervals 56 248 0)
 (buffers 976 12)
 (heap 1024 25332 985))