Full error message:

Error running timer ‘jit-lock-stealth-fontify’: (wrong-type-argument timerp [t nil nil nil nil nil nil nil nil])

To reproduce:

1. Build latest trunk

2. Paste the following into some file, lets call it custom-test.el (I couldn’t figure out a better way to do this)

3. Then load this file and open another file which would trigger fortification:

$ emacs -q --load ./custom-test.el ./lorem.py

4. Check *Messages*, the error should come up every second now

I’ve also gotten the following related errors (but don’t know how to precisely trigger them):

- File mode specification error: (wrong-type-argument timerp [t nil nil nil nil nil nil nil nil])

- custom-initialize-reset: Wrong type argument: timerp, [t nil nil nil nil nil nil nil nil]

Reverting to right before commit 50765f3f511d467ff024dda7c84530c759253d18:

Author: Lars Ingebrigtsen <larsi@gnus.org>
Date:   Tue Aug 31 03:04:22 2021 +0200

    Make run-at-time try harder to run at integral multiples

    * lisp/emacs-lisp/timer.el (timer): Add new slot integral-multiple.
    (timerp): Adjust.
    (timer-event-handler): Recompute the delay if requested
    (bug#39099).
    (run-at-time): Mark the timer as recomputable if given a t
    parameter.

    * src/keyboard.c (decode_timer): Adjust.

Seemed to fix it (but I wasn’t able to figure out exactly what about this PR caused the error).