Since I upgraded to MediaWiki 1.30, I get those notices:
PHP Notice: ob_end_flush(): failed to delete and flush buffer. No buffer to delete or flush in mediawiki-1.30.0/includes/filerepo/FileRepo.php on line 1612
This happens when someone access Special:UploadStash when a file failed to upload due to a warning.
Looking at the code, it seems to be caused by the code added for T172851
Using PHP 7.0.19-1