Eli Zaretskii <eliz@gnu.org> writes:

>> From: Andreas Politz <politza@hochschule-trier.de>
>> 
>> (let ((d "/tmp/\303\204")) ;; utf-8 for german umlaut "A 
>
> This makes d a unibyte string:
>
>   (setq d "/tmp/\303\204")
>   "/tmp/\303\204"
>
>   (multibyte-string-p d)
>     => nil
>
> Why would one do such a thing in the first place?

OK.  The thing is that Emacs does it.