Emacs crashed when visiting specific pages in a browser using EXWM.
Turns out this was triggered when the buffer was renamed accordingly.
During redisplay, the crash happens for certain strings which are set as
`mode-line-format`.  I created an example to reproduce this with a stock
Emacs configuration, see the attached elisp file.  For me, this only
triggers when the window is large enough, I suspect smaller windows will
prevent the problematic part of the mode-line-format string to be
processed.

To reproduce:

1. Start Emacs using 'emacs' (not tested with 'emacs -Q')
2. Maximize Frame
3. M-x ielm
4. (load "/path/to/attached/break-emacs.el")
5. M-x crash-test-mode-line-format
6. Observe crash

The output of 'bt full' is attached in 'gdb.txt'

In GNU Emacs 27.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.27, cairo version 1.16.0)
Windowing system distributor 'The X.Org Foundation', version 11.0.12010000
System Description: NixOS 20.09 (Nightingale)

Configured using:
 'configure
 --prefix=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2
 --bindir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/bin
 --sbindir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/sbin
 --includedir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/include
 --oldincludedir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/include
 --mandir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/share/man
 --infodir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/share/info
 --docdir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/share/doc/emacs
 --libdir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/lib
 --libexecdir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/libexec
 --localedir=/nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/share/locale
 --disable-build-details --with-modules --with-x-toolkit=gtk3 --with-xft
 --with-cairo'

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY
INOTIFY LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD JSON
PDUMPER GMP

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

Load-path shadows:
/run/current-system/sw/share/emacs/site-lisp/site-start hides /nix/store/2csx4hz6ab0i6d5h4b7z1afwsydbx02n-emacs-packages-deps/share/emacs/site-lisp/site-start
/run/current-system/sw/share/emacs/site-lisp/site-start hides /nix/store/l8wil534cr2bp339s27k6p2i8zyi5y6k-emacs-27.2/share/emacs/site-lisp/site-start

Features:
(shadow sort mail-extr emacsbug sendmail notmuch notmuch-tree
notmuch-jump notmuch-hello wid-edit notmuch-show notmuch-print
notmuch-crypto notmuch-mua notmuch-message notmuch-draft
notmuch-maildir-fcc notmuch-address notmuch-company notmuch-parser
notmuch-wash diff-mode easy-mmode coolj notmuch-query goto-addr
thingatpt icalendar diary-lib diary-loaddefs cal-menu calendar
cal-loaddefs notmuch-tag crm notmuch-lib notmuch-compat pcase hl-line
message rmc puny dired dired-loaddefs format-spec rfc822 mml mailabbrev
gmm-utils mailheader mm-view mml-smime mml-sec epa derived epg
epg-config gnus-util rmail rmail-loaddefs mail-utils
text-property-search time-date smime dig mm-decode mm-bodies mm-encode
mailcap mail-parse rfc2231 rfc2047 rfc2045 mm-util ietf-drums mail-prsvr
package easymenu browse-url url-handlers url-parse auth-source cl-seq
eieio eieio-core cl-macs eieio-loaddefs password-cache json subr-x map
url-vars seq byte-opt gv bytecomp byte-compile cconv cl-loaddefs cl-lib
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 replace newcomment text-mode elisp-mode
lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow isearch
timer select scroll-bar mouse jit-lock font-lock syntax facemenu
font-core term/tty-colors frame minibuffer 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 composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray
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 threads dbusbind
inotify dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 83001 9776)
 (symbols 48 9884 1)
 (strings 32 28953 2047)
 (string-bytes 1 1044840)
 (vectors 16 15878)
 (vector-slots 8 199051 14998)
 (floats 8 27 44)
 (intervals 56 319 0)
 (buffers 1000 14))