Steps to reproduce

1. Create a file with a illegal character. At our example we got line 38 at the document: $wgFlaggedRevsWhitelist = [ 'الص�?حة_الرئيسية' ];
2. Now go to the file in diffusion.
3. At "View Raw File" you can see the character.
4. Without raw, diffusion won't show the character: $wgFlaggedRevsWhitelist = [ ]; (see here: https://phabricator.wikimedia.org/diffusion/OMWC/browse/master/wmf-config/flaggedrevs.php%3B5726e947e7998adf64503cb51ba30879c7fb4e92%2438)
5. Now disable syntax-highlighting off. The file will get empty now.

Original report by @valhallasw (https://phabricator.wikimedia.org/T144672) I reproduced it, with the following

Versions

phabricator
    0eb5a80e7b689fa9261e3b6ee7640664bfe14a5f (Fri, Sep 2) 
arcanist
    9e82ef979e8148c43b9b8439025d505b1219e213 (Thu, Aug 25) 
phutil
    c8b76485ef845a40af96c132c16e0f28f145bdd2 (Fri, Sep 2)

locally, without any additional extensions installed (default setup)