**Steps to reproduce:**
Unclear.
Looking at downstream logs, running an old Phorge/Phabricator version but looking at upstream code unchanged since March 2018 I'd still expect this to happen:
```
[2023-08-21 11:47:15] EXCEPTION: (RuntimeException) mb_convert_encoding(): Illegal character encoding specified at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(), ava(), phabricator(), translations(), wmf-ext-misc()
#0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer, array) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
#1 <#2> mb_convert_encoding(string, string, string) called at [<phabricator>/src/applications/files/document/PhabricatorTextDocumentEngine.php:73]
#2 <#2> PhabricatorTextDocumentEngine::loadTextData(PhabricatorDocumentRef) called at [<phabricator>/src/applications/files/document/PhabricatorSourceDocumentEngine.php:29]
#3 <#2> PhabricatorSourceDocumentEngine::newDocumentContent(PhabricatorDocumentRef) called at [<phabricator>/src/applications/files/document/PhabricatorDocumentEngine.php:133]
#4 <#2> PhabricatorDocumentEngine::newDocument(PhabricatorDocumentRef) called at [<phabricator>/src/applications/files/document/render/PhabricatorDocumentRenderingEngine.php:117]
#5 <#2> PhabricatorDocumentRenderingEngine::newDocumentView(PhabricatorDocumentRef) called at [<phabricator>/src/applications/diffusion/controller/DiffusionBrowseController.php:195]
#6 <#2> DiffusionBrowseController::browseFile() called at [<phabricator>/src/applications/diffusion/controller/DiffusionBrowseController.php:48]
#7 <#2> DiffusionBrowseController::handleRequest(AphrontRequest) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:284]
#8 phlog(RuntimeException) called at [<phabricator>/src/aphront/handler/PhabricatorDefaultRequestExceptionHandler.php:41]
#9 PhabricatorDefaultRequestExceptionHandler::handleRequestThrowable(AphrontRequest, RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:751]
#10 AphrontApplicationConfiguration::handleThrowable(RuntimeException) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:296]
#11 AphrontApplicationConfiguration::processRequest(AphrontRequest, PhutilDeferredLog, AphrontPHPHTTPSink, MultimeterControl) called at [<phabricator>/src/aphront/configuration/AphrontApplicationConfiguration.php:204]
#12 AphrontApplicationConfiguration::runHTTPRequest(AphrontPHPHTTPSink) called at [<phabricator>/webroot/index.php:35]
```