Eli Zaretskii <eliz@gnu.org> writes:

>> The behavior differs slightly from those seen earlier because the
>> timer list contains only three timers when it tries to add another
>> one.  Still the conjecture that we try to add a timer while checking
>> timers has been proven.
>
> I'd love to see a C-level backtrace from that situation, because I'm
> not really sure what exactly happens and how.

This is reproducible from

    emacs -Q -L .../w3m -l w3m -f w3m

where .../w3m is a checkout of https://github.com/ecbrown/emacs-w3m.
Backtrace attached.  Martin's patch of #143 applied and breakpoint
set with:

    break Fsignal if (((intptr_t)Qerror) == ((intptr_t)error_symbol))

(the breakpoint needs to be set only after w3m has started up though)