Here is an example output from Slack, even though weirdly enough nothing 
seemed to trigger the error:

|======================================================================<br 
/>
1 -&gt; (comint-output-filter #&lt;process Shell&gt; &quot;[05/12/20, 
13:06:14:636] info: [DESKTOP-SIDE-EFFECT] Update from desktop for keys 
&nbsp;[\&quot;settings\&quot;]&nbsp;<br />
&quot;)(:comint-pmark nil)<br />
| 2 -&gt; (set-marker #&lt;marker at 37370 in *Async Shell Command*&gt; 
37469)(:comint-pmark (#&lt;marker at 37370 in *Async Shell Command*&gt; 
. #&lt;marker at 37469 in *Async Shell Command*&gt;))<br />
| 2 &lt;- set-marker: #&lt;marker at 37469 in *Async Shell 
Command*&gt;(:comint-pmark (#&lt;marker at 37469 in *Async Shell 
Command*&gt; . #&lt;marker at 37469 in *Async Shell Command*&gt;))<br />
| 2 -&gt; (set-marker #&lt;marker at 37469 in *Async Shell Command*&gt; 
37566)(:comint-pmark (#&lt;marker at 37469 in *Async Shell Command*&gt; 
. #&lt;marker at 37469 in *Async Shell Command*&gt;))<br />
| 2 &lt;- set-marker: #&lt;marker at 37566 in *Async Shell 
Command*&gt;(:comint-pmark (#&lt;marker at 37469 in *Async Shell 
Command*&gt; . #&lt;marker at 37566 in *Async Shell Command*&gt;))<br />
| 2 -&gt; (ansi-color-process-output &quot;[05/12/20, 13:06:14:636] 
info: [DESKTOP-SIDE-EFFECT] Update from desktop for keys 
&nbsp;[\&quot;settings\&quot;]&nbsp;<br />
&quot;)(:comint-pmark (#&lt;marker at 37469 in *Async Shell Command*&gt; 
. #&lt;marker at 37566 in *Async Shell Command*&gt;))<br />
| 2 &lt;- ansi-color-process-output: nil(:comint-pmark (#&lt;marker at 
37469 in *Async Shell Command*&gt; . #&lt;marker at 37566 in *Async 
Shell Command*&gt;))<br />
| 2 -&gt; (set-marker #&lt;marker (moves after insertion) at 37566 in 
*Async Shell Command*&gt; 37566)(:comint-pmark (#&lt;marker at 37469 in 
*Async Shell Command*&gt; . #&lt;marker at 37566 in *Async Shell 
Command*&gt;))<br />
| 2 &lt;- set-marker: #&lt;marker (moves after insertion) at 37566 in 
*Async Shell Command*&gt;(:comint-pmark (#&lt;marker at 37469 in *Async 
Shell Command*&gt; . #&lt;marker at 37566 in *Async Shell 
Command*&gt;))<br />
1 &lt;- comint-output-filter: #&lt;marker (moves after insertion) at 
37566 in *Async Shell Command*&gt;(:comint-pmark nil)<br />|

Because I am on the testing branch of Debian, is there any chance Emacs 
got updated and this bug fixed?

On 05/05/2020 19:33, Noam Postavsky wrote:
> Jacob Lagares Pozo <jlagarespo@iebesalu.cat> writes:
>
>> If I run it without your patches, it works surprisingly just fine (I
>> noticed the original errors pop up most commonly on Slack I guess
>> because it prints a lot more?), whereas if I evaluate said patches,
>> this is the output of the trace buffer:
> I don't see anything unexpected in the trace either.
>
>> I am not sure what does this mean, perhaps it is some special
>> character Slack uses for logging that messes with those markers, I
>> don't know. Maybe I could try printing all of the ASCII characters
>> sequentially and see what happens.
> Well, ideally we would want a trace from something that does trigger the
> error.
>
>> Regardless, I'm still not entirely sure what your code is doing anyway.
> It's basically just printing out the values of the markers, so we might
> hopefully notice when they get changed in an unexpected way.