This only rears its head when the Fast String Search extension is not present. When it is present, the extension swallows the error.
ErrorException from line 120 of includes/libs/ReplacementArray.php: Object of class ResourceLoaderContext could not be converted to string #0 includes/libs/ReplacementArray.php(120): MWExceptionHandler::handleError(integer, string, string, integer) #1 languages/Language.php(3176): ReplacementArray->replace(string) #2 languages/classes/LanguageAr.php(47): Language->transformUsingPairFile(string, string) #3 includes/WebRequest.php(318): LanguageAr->normalize(string) #4 includes/WebRequest.php(345): WebRequest->normalizeUnicode(string) #5 includes/WebRequest.php(363): WebRequest->getGPCVal(array, string, NULL) #6 includes/resourceloader/ResourceLoaderContext.php(162): WebRequest->getVal(string) #7 includes/resourceloader/ResourceLoader.php(1544): ResourceLoaderContext->getLanguage() #8 includes/resourceloader/ResourceLoader.php(1485): ResourceLoader::createLoaderQuery(ResourceLoaderContext, array)